IT'S EARLY Tuesday night at Whitey's Restaurant in Arlington. Half a dozen guitar cases are stuffed under the tiny stage and microphone levels are being quickly set. It's open mike night, and the boisterous crowd is about to be treated to an a cappella version of "Lord, It's Hard to Be Humble," an inspired arrangement of the Who's "Won't Get Fooled Again," a yodeling cowboy, a smattering of Kingston Trio brio and not one, but two, stabs at "16 Tons."

And that's only the first six musicians.

Open mike nights, held at nearly two dozen bars in the Washington area, are sort of like high-school talent shows for grown-ups. One night a week, a bar will open up its stage to any closet performers who are willing to share their talents with the audience. For some, that means crooning a Neil Young song on a battered guitar. For others, it's doing the comedy routine that keeps their friends in stitches. Still others use open mikes as a great low-pressure way of trying out their own original material.

Open mikes fill a void in a city much maligned for its music scene. "The role of the open mike is to break through the cliques," says Ace Smith, a singer-guitarist who plays host to two sessions a week. He says that since club owners don't like to take chances on unproven acts, open mikes are one of the few ways a musician can get experience.

"A lot of the people who play here are just starting out," says Alan Weinberg, a Montgomery County school teacher who hosts the Sunday night open mike at Gallagher's III in Gaithersburg. "They'd like to play music professionally part-time and need a forum to practice and play over a mike."

Mike Moore is one such open-miker. He's been going to Gallagher's III for two years, working on his material and building up his confidence. "The first time I sang here I was so scared my voice had a built-in vibrato," Moore says. He's a bit smoother now and playing at an open mike has given him valuable feedback on his own songs, many of which he'll be playing at Gallagher's III on August 27, his first professional gig.

The people who host open mikes, musicians themselves who normally appear elsewhere throughout the week, enjoy watching the amateurs progress. Monday night at Alice's Mr. Henry's in Alexandria has its own group of regulars who return week after week. Says host Vicki Pratt, "It's nice to see them get better and better."

For every aspiring musician who uses an open mike as a chance to polish skills, there's one who's a little less professional and just wants to blow off a bit of steam and play for someone other than the mirror. "Audiences will support anyone for 10 to 15 minutes, regardless of talent," says Weinberg. "Proficiency doesn't matter; it's how much the performer is enjoying himself." And, with a few exceptions, 10 to 15 minutes is about how long a performer gets to win the crowd over.

Everyone's in the same boat at an open mike, quickly dashing off a few songs in hopes that the crowd will respond. This engenders a supportive atmosphere of musical camaraderie. Established musicians will do a few numbers, listen to some of the amateurs and offer advice or criticism. Indeed, at some open mikes, the best music happens after the microphones have been turned off, when the remaining musicians play together in impromptu jam sessions.

For the bars that host them, open mikes are good business, because most performers bring several drink-buying friends for moral support. It also provides club owners with a low-risk way of auditioning new acts. And, since they don't have a cover charge, open mikes are a cheap night out for those who just want to listen and be treated to an incredibly diverse range of music. You may have to sit through a few excruciating acts but before the night is over you'll probably hear someone who's "good enough to be professional." Like weather in Washington, if you don't like what you're hearing at an open mike, stick around, it'll change.

While most open mikes are meant for singers who accompany themselves on acoustic guitars, jazz and blues musicians also have outlets in Washington for their talents. A few jazz clubs, including Philly's Finest and One Step Down, hold afternoon "workshops." These are a bit more structured than the classic open mike and often attract professional musicians who drop by when playing in town. The Grog and Tankard on Wisconsin Avenue holds a blues jam Thursday nights that brings some accomplished bluesmen out of the woodwork. At Mr. Henry's in Adams Morgan, you can bring your backing music on a cassette tape and sing along over the sound system. At the Ibex V.I.P. Lounge on Georgia Avenue, amateur entertainers compete for cash prizes in everything from singing to dance.

Stand-up comedians are an increasing presence at open mikes, too, posing a challenge to the evenings' hosts. "The hardest thing is telling comedians to stop," says Phil Duarte, host of Food for Thought's Monday open mike. "With a singer you can put one finger up and say 'One more song.' It's tough telling a comedian 'One more joke.' " Comedians are welcomed at most open mikes, though.

Though the key to a successful open mike is variety, there are some constants. Songs by Bob Dylan are understandably popular, since proficiency can take a back seat to feeling and emotion. James Taylor and John Prine -- who himself cut his teeth on open mikes -- are also well represented. (At any given acoustic open mike, Prine's "Spanish Pipe Dream" has a better-than-even chance of being played.) And it's a safe bet that selections from Joni Mitchell's oeuvre will be performed by female musicians.

This doesn't stop open mikes from being one of the best musical bets in Washington. For the audience, it's a chance to hear an incredibly diverse range of music. For the performers, it's a chance to get out of the basement and work in front of living, breathing people. If only for 15 minutes.

OPEN SEASON

Every open mike has its own atmosphere so it's best to visit a few to find the ones at which you'll feel most comfortable. (Call ahead to make sure they're having one since some open mikes come and go with alarming frequency.) When they've found one you like, it's a good idea to arrive early since the sign-up sheets can be filled quickly.

The technical skills of open-mike players vary considerably but since the most important thing is to have a good time, enthusiasm is often more important than virtuosity. However, open-mike hosts will expect you to have some structure to your act. Be ready when they ask you to perform, finish within your allotted number of minutes or songs, don't be too nervous and, in the words of more than one host, "Don't be snotty."

Here are some of Washington's more established open-mike nights: SUNDAYS 219 BASIN ST. LOUNGE -- 219 King St., Alexandria, 549-1141. Brook's Hot Jazz afternoon jam session, 1 to 6 p.m. As at most jazz jam sessions, the skill level Sundays at 219 is very high. Musicians interested in jamming should arrive early and talk to someone in the band. MR. EARLE SUPPER CLUB -- 4226 Wisconsin Ave. NW. 686-1000, John Harbison's Open Music Jam. A spacious stage makes this open mike popular with fledgling bands. Single musicians and comedians also perform. Interested open-mikers should talk to host John Harbison. GALLAGHER'S -- 3319 Connecticut Ave. NW. 686-9189. Hosted by Steve Erwin. Musicians usually play three songs -- about 15 minutes -- depending on how many have come to play. Styles range from folk to bluegrass to Irish to light rock. This open-mike night has a very nice, supportive atmosphere. GALLAGHER'S III -- 16533 S. Frederick Rd., Gaithersburg. 977-9000. Hosted by Alan Weinberg. The open mike at Gallagher's III starts early, at 8, but that's so host Alan Weinberg can get everything in. After the last four-song set is played at 11:30, the PA system is turned off, the guitars are pulled out and the open-mikers have an informal acoustic jam session. Styles at the open mike itself are varied, but Gallagher's III seems to have more country music than many others. "A lot of people come in to work out original material or just to get used to playing in front of a real audience," says Weinberg. GROG & TANKARD -- 2408 Wisconsin Ave. NW. 333-3114. Hosted by Bob Becker and Steve Schofield. Singer-songwriters should check out the Grog and Tankard's open mike night. It is, says co-host Bob Becker, "a good outlet for original material." You'll hear a lot of it at the Grog and Tankard, played mostly by young musicians. Indeed, with several universities nearby, the crowd is one of the youngest in town. ONE STEP DOWN -- 2517 Pennsylvania Ave. NW. 331-8863, Jazz Workshop with the Lawrence Wheatly Quartet. One Step Down features jazz five nights a week and on Saturday and Sunday afternoons from 3:30 to 7:30 hosts a jazz workshop. Interested musicians just add their name to a sign-up sheet. MONDAYS ALICE'S MR. HENRY'S -- 1319 King St., Alexandria, 836-3377. Hosted by Vicki Pratt. This open mike starts around 9. and runs till 1. Guitarist Pratt opens the evening with a few of her originals before turning things over to the dozen acts that perform each Monday. The styles tend toward acoustic pop-folk, but a few bands have been known to lug their equipment in. Keyboard players interested in doing a few numbers should check out Alice's: It's one of the few open mike venues that has a baby grand piano. EUGERTHA'S RESTAURANT & LOUNGE -- 6817 Georgia Ave. NW. 291-1991. New Music Workshop, hosted by Greg Lamont. This new jazz workshop starts at 9 with a few sets from organist Greg Lamont. While Lamont won't discourage anyone from playing, the workshop's designed for skilled musicians who want to experiment with new forms and learn from one another. FLANAGAN'S -- 7706 Woodmont Ave., Bethesda, 986-1007, World Folk Music Association Open Mike, second Monday of each month. Hosted by the guitar duo Side By Side; usually features folk and Irish music. FOOD FOR THOUGHT -- 1738 Connecticut Ave. NW. 797-1095. Poets, actors, comedians as well as the requisite guitar-playing singers make this arguably the most interesting open mike in town. Think of it as an alternative, free-form open mike night. "We've even had full bands with drums come in," says host Phil Duarte. There are a few rules, though: No Joni Mitchell. Also banned is Little Feat's "Willin'" and the Grateful Dead's "Friend of the Devil." "No one gets the chords right," says Duarte. GALLAGHER'S -- 3319 Connecticut Ave. NW. 686-9189. Open mike with Ace Smith. A few accomplished finger-picking bluegrass players are regulars at this open mike, though before the night is through many styles will be represented. Like its Sunday night counterpart, this is a low-pressure, fun open mike. TUESDAYS MR. HENRY'S -- 1836 Columbia Rd. NW. 797-8882. Hosted by Tom Saputo. The Tuesday and Wednesday open-mike nights at Mr. Henry's in Adams-Morgan are for people who want to be stars. The focus is on R&B and soul and all you have to bring is your voice. Versatile keyboardist Tom Saputo will accompany you, provided you have brought him sheet music or the tune's part of his large repertoire. A few regulars -- Saputo says nearly 80 percent of the singers return each week -- bring in their backing tracks on cassette tapes. For $4 Tony the soundman will make a cassette of your performance. The crowd is one of the largest in open-mikedom, but supportive. WHITEY'S -- 2761 Washington Blvd., Arlington, 525-9825. Hosted by Bob Ellis. It must be the broasted chicken or cheap beer that works its magic on the Tuesday night crowd at Whitey's Restaurant. "They're not rude or nasty," says Ellis. "They can be indifferent, but they're the best crowd for an open mike." Indeed, if you can penetrate the din, chances are the audience will be clapping and singing along. Performers usually play four to six songs; styles range from country to rock 'n' roll. WEDNESDAYS GALLAGHER'S PENN STATION -- 637 Pennsylvania Ave. SE. 546-8368. Ubiquitous open mike host Ace Smith organizes the Gallagher's Penn Station session into 15-minute blocks. "If a duo comes in, or an act that I know has rehearsed, I might give them a little more time," he says. The styles are varied and Smith says he'd like to see it become even more eclectic. IBEX VIP LOUNGE --

5832 Georgia Ave. NW. 726-1800. Open Mike Talent Search hosted by Nathan Heathman. Wednesday at the Ibex is Washington's own version of "Star Search." Amateur entertainers (the club disallows anyone who's cut a record) compete for prizes in the categories of vocalist, band, comedy and dance. The audience picks the winners, with $50 going to first place, $25 to second and $10 to third. Winners then perform in quarterly semifinals before advancing to the finals in December and a grand prize of $1,000. Keyboardist Nathan Heathman, relying on his large repertoire of top 40 and R&B songs, will accompany vocalists. Backing tapes are also acceptable. Signup is from 8 to 9:30; the first act hits the stage at 8:30. MR. HENRY'S -- 1836 Columbia Rd. NW. 797-8882. See Tuesday listing. JIMMY McPHAIL'S GOLD ROOM -- 1122 Bladensburg Rd. NE. 399-1444. This jazz workshop starts at 9 with a set by the Maurice Robertson Quartet. Musicians should just come by and, says owner Jimmy McPhail, "someone will take care of them." He adds, "There's a real need for places for young entertainers to play." QUINCY'S -- 5444 Columbia Pike, Arlington. 671-2774. This relatively new open mike starts at 8 and is hosted by Steve Erwin. Featured artists perform later in the evening. THURSDAYS BOSCO'S -- 8210 Piney Branch Rd., Silver Spring, 588-4440. Hosted by Eddie Becker. Bosco's open mike, held every other Thursday, is one of the more eclectic in town, from John Prine covers to Buddy Hollyesque originals to strange comedy. Heavy-metal bands, complete with drum sets, have been known to show up. GROG & TANKARD -- 2408 Wisconsin Ave. NW. 334-3114. The Smut Brothers, a five-piece blues band led by hot guitarist Mike Melchione, host this quality jam session, the Grog's weekly "Blues Jam." The music starts at 10:30, when the Brothers open up with an hour or so of R&B and Southern blues. Musicians who want to play should arrive around 10 and speak to someone in the band. About 75 percent of the people who play are regulars. Complete bands often show up, though Melchione will match up lone musicians so everyone gets a chance to play. While Melchione likes to hear beginners, the skill level of most musicians at this open mike is high. "You should be able to hold your own up on stage," he says. TUCSON CANTINA -- 2605 Connecticut Ave. NW. 462-6410. Hosted by Chris Burke. Generally features acoustic-oriented music. Each act plays about 20 minutes and host Chris "D.C. Cab" Burke often uses the evening to audition prospective musicians for later dates. Music tends to be acoustic-oriented light rock. The Tucson Cantina boasts one of the finest P.A. systems on the open-mike scene. FRIDAYS POTTER'S HOUSE -- 1658 Columbia Rd. NW. 232-5483. Hosted by Alice Benson. Customers at the Potter's House -- one of the oldest coffee houses in the country -- get to choose the new musicians they'll be hearing. The Potter's House holds open auditions the first Fridays of February, June and October. Acoustic folk guitar is most prevalent, but host Alice Benson says jazz, bluegrass and rock are also represented. Customers vote on each act and those with the highest scores are invited back. The next open audition is October 2. TAKOMA CAFE -- 1 Columbia Ave., Takoma Park, 270-2440. Hosted by David Pollick. "Open Mike" at the Takoma Cafe, the Washington area's own bohemian coffee house, is a bit of a misnomer: There isn't actually a microphone. There is, however, a very attentive audience listening to a wide range of folk music. The Cafe is a cooperative, and many of the workers you'll see there are members who volunteer at least three hours of their time each month. A group-house atmosphere permeates The Cafe and guitars are strummed as much on the porch outside as on the stage inside. While there is no cover charge, a jar is passed around for those interested in donating. Performers are usually allowed four songs. SATURDAYS ONE STEP DOWN See Sunday listing. PHILLY'S FINEST -- 1601 Rhode Island Ave. NE. 529-2380. Afternoon jam session with Keith Holmes, 4 to 8. While the first floor of Philly's Finest is a run-of-the mill fast-food joint specializing in steak-and-cheese subs, the upstairs lounge has a groove all its own. Keith Holmes leads a quality backing band that leans toward straight-ahead jazz. Musicians, vocalists, and even comedians are welcome to add their name to a list and take part in the workshop. "You get to call your own tunes here," promises Holmes.

Freelance writer John Kelly can sometimes be seen at open mike nights as half of the Neverly Brothers.