With the opening of "Carmelian" last night, Washington took custody, for three weeks, of an awesome store of musical comedy experience, invention and know-how. They go by the names (Word Illegible) and Lane.

Those who don't regard the mere presence in the Kenndy Center Opera House of two such legends as sufficient return on an $8.50-$18.50 investment will be pleased to hear that the legends in question are not yet ready to be put under glass in some theatrical museum.

Although 'Carmelina" certainly has its dumb side, it offers many of the old-fashinged pleasures one would expect from a team of writers whose cumulative credits (if we add colibrettist Joseph Stein to the reckoning) include "Finian's Rainbow," "Brigadoon," "My Fair Lady," "Camelot," "Fiddler on the Roof" and "On a Clear Day You Can See Foreever."

And it has Georgia Brown, who somehow manages to seem just as ideally cast here, in the pictureaque Italian hinterland, as she did in the grimy 19th-century London of "Oliver."

Miss Brown is, putting it conservatively, a knockout. No matter how far back in the hall you are, she gets to you. And there is never an awkward transition from speech to song, that strange moment when some performers seem to be plugging in a new set of tonsils and purging from their minds and limbs all memory of how real people comport themselves.

Carmelina Campbell, the character made memorably by Brown's smoky voice and artful eyebrows, is a professional, full-time widow. For nearly 20 years she has been in a state of mourning for the American Army lieutenant who married her and then rushed off to a glorious death driving the Nazis from Italy.

That, at any rater, is what her daughter, her priest, her would-be lover and the townspeople think. The bald trutn is that she never was married to anyone; she lifted her name from a soup can and has been living on hcild-support checks cajoled out of three different ex-G.I. lovers. And now her cover is about to be blown -- the threesome is coming back to the little town of San Forino, "somewhere is Italy," for a reunion.

This potent premise (based on a true story that was the foundation for the 1969 movie, "Buona Sera, Mr. Campbell") has hardly been plumber to its full depths, but when Brown unlooses one of the elegantly crafted songs Alan Jay Lerner and Burton Lane have written for her, everything about the show seems as right as she.

When she is not singing one of those no-holds-barred Mediterranean melodies -- worse yet, when she is not singing at all or, perish the thought, not even onstage -- "Carmelian" moves along in fits and starts.It seems, at this stage of its gestation, to be deep in the throes of a pre-Broadway identity crisis.

At times the show betrays aspirations to play La Scala -- especially, when costar Cesare Siepl is applying his rich, formidable baritone to a love song called, for the shameful want of a better idea, "It's Time for a Love Song." I don't quarrel with the sentiment; it may well be time for a love song -- who would know better than an old master like Alan Jay Lerner? -- but surely it is time for a love song with more to say for itself.

It's pretty tune -- very -- but the net effect is reminiscent of the sort of inanity Alan Jones used to be called on to sing for "dramatic relief" in the middle of a Marx Brothers movie. He usually would get stoned somewhere in the second verse. Indeed, in its second reprise the song (which has been extensively promoted in radio ads for "Carmelina") is subjected to a measure of ridicule -- a door is slammed in Siepi's face before he can even make his way through the opening line. But it wouldnot be a gross slander to hint that possibly the authors are trying here -- just a little -- to have it both ways.

Siepi has an added handicap to deal with -- the basic incompatibility of operatic singing and musical comedy singing, which mix about as well sometimes as tomato sauce and apple pie. In "South Pacific," at least, Ezio Pinza and Mary Martin were supposed to be a strange match themselves, born as they were on the opposite sides of the sea, and as different, you will recall, as people could be.Siepi's role, as the town tavernkeeper and Carmelina's frustrated admirer, provides him with no such license to be an anomaly. And to make matters still more difficult, when he launches into song the words and rhyme schemes tend to lose themselves in the roundness of his notes, as though someone -- the singer or his audience -- had suddenly plunged underwater.

Strangely, but happily, Siepi looks more at home in some of his non-singing moments, including several deftly handled bits of romantic repartee (SIEPI, to Brown: I want to make love to you. BROWN: You want to make love to me? For how long? SIEPI, dumbfounded: I don't know. BROWN, after a beat: No, I mean how long have you wanted me.)

At other times, particularly after the three mostly indistinguishable ex-G.I's enter the proceeding toward the end of Act One, "Carmelina" develops an itch to trun farcical, but without the care or commitment that makes farce funny. In the long and largely unstudied history of comic scenes involving multiple lovers simultaneously hiding in one woman's domicile, "Carmelina's" Entry -- which brings down the first-act curtain -- will not rank high.

But somewhere between its uncomfortable extremes, if the authors and director Jose Ferrer can get their bearings, lies a winning musical on the various ways in which humans employ concealment, misunderstanding and fakery to thwart their own prospects for happiness. (An across-the-board decision to raise the I.Q. of "Carmelina's" characters by about 30 poinmts would be a reasonable first step toward finding that musical.)

"A musical is a popular are form," Lerner has written in his amiable memoir "The Street Where I Live." "I do not believe there is such a thing as an avante-garde musical: It is a contradiction in terms."

Some theatergoers may have come to a slightly different conclusion lately -- that perhaps there would never again be a non-avante-garde musical. For those who have thought such a thing, and found the thought unsettling, "Carmelina" is a refreshing breath of stale air. CAPTION:

Picture, Georgia Brown, right, and Grace Keagy, by Douglas Chevalier