Robert Benton's "The Late Show," opening today at the K-B Cinema, is a modestly conceived but surprisingly satisfying entertainment, a private-eye melodrama that looks and sounds up-to-date while respecting the traditions and conventions of the genre.

Art Carney's Ira Welles, the '40s private-eye hero grown old, is reticent, solitary, stable. Slowed by age and sickness, he remains a formidable relic, skillful and resourceful and proud.

Lily Tomlin's Margo Sperling, the client who galvanizes Ira into winded but gallant and effective action, is his temperamental opposite: voluble, gregarious, neurotic, an L.A. fruitcake stuffed with fashionable cant yet likable despite all her pretensions.

Ira and Margo are humorously reconstituted versions of the intrepid detective and the designing woman client who puts him simultaneously on the spot and on his mettle. They're an irresistible odd couple: winning losers, as appealing in their own unlikely fashion as Rockey and Adrian in Sylvester Stallone's "Rocky." Contradictory as they appear, Ira and Margo have something to offer each other. Not everything, but something. Benton has kept the relationship comic and tentative; one barely thinks of Ira and Margo as a romantic match, particularly in view of Ira's precarious physical condition.

Warning to the chase, Margo compares them to Nick and Nora Charles, but this is obviously her delusion, not the author's. Ira can't place the names, and when Margo tries to enlighten him, Benton improves on his own joke by revealing that she hasn't got quite the right Nick and Nora Charles. Ira and Margo are probably closer to Holmes and Watson, although in their case the professional detective has Watson's reserve and the amateur has Holmes' egocentric excitability.

The developing emotional rapport between these characters may be more responsible for the film's popularity than Benton's snappy script and deft direction. Hostility and treachery seem to have become so prevalent on the contemporary screen that one can feel audiences go slightly giddy when confronted by depictions of genuine affections. It happens when Rocky is courting his girl, when Nicol Williamson's Holmes apologizes to Robert Duvall's Watson in "The Seven-Per-cent Solution," when William Katt escorts Sissy Spacek to the prom in "Carrie," a movie that serves as a touchstone to audience uncertainties.

Ira and Margo get mixed up with a vicious bunch playing for keeps - before the movie runs its brisk 94-minute course, eight homocides have been depicted or accounted for - but it's a compact bunch, and Benton orders their vicious behavior with a precision and discretion that eluded Robert Altman in "The Long Goodbye." (Altman fronted as producer for "The Late Show," and Benton freely acknowledges his influence.) Benton's villians belong to the same contemporary Los Angeles underworld of gangsters, thugs and chiselers, but they're more effective for being condensed and toned down.

In fact, Benton has invented a corrupt gang as small yet distinctive as the one that pursued the Maltese Falcon. The ruthless figures are incisively funny: Eugene Roche's Mr. Big, a deceptively affable, phligmatic receiver of stolen goods named Birdwell; Bill Macy's sleazy, ferret-faced middleman Charlie Hatter; John Considine's Lamar, a handsome, narcissistic, dimwit bodyguard, so childishly brutal that he seems to expect the people he roughs up to have no hard feelings, since he has no hard feelings himself; Joanna Cassidy's adulterous, elusive Mrs. Birdwell, always enticing and never to be trusted.

The main thing to guard against at "The Late Show" is entertaining skyhigh expectations. Think of it as a nifty small movie and you shouldn't be disappointed. It might not have made much of an impression in a more abundant movie season. As far as cinematic cuisine goes, "The Late Show" is really more of an appetizer than a mian course. However, at this moment the apparent appetizers - low-budget sleepers like "The Late Show," "Rocky" and "Carrie" - are probably keeping the business from croaking on such main courses as "King Kong," "The Last Tycoon" and "Fellini's Casanova."

Typically, the plot seems tense and amusing while it's unfloding and something of a muddle after the hero considerately summarizes killings and motives for the denouement. The recapitulations in detective thrillers are rarely satisfying and never as much fun as the developing mystery and the hero's encounters and byplay with the other characters. My own inclination is to give the mystery writer credit as long as the parties with the most blood on their hands and wickedness in their hearts remain dimly identifiable - and "The Late Show" satisfies that loose requirement.

Benton and cinematographer Chuck Rosher have given the picture an admirably clean, absorbing lock that accentuates the humor in the situations, the dialogue and the performances. "The Late Show" looks intuitively right from the opening image, a shot the same generation and reflect a simmanuscript page of his would be memoirs, entitled "Naked Girls and Machine Guns." It's a pleasantly disarming, unemphatic shot, in contrast to the big typewriter key downbeat that began the movie version of "All the President's Men," signaling Important Movie Ahead.

Benton seems to know from the start how much weight his material can support, and his direction is at once more relaxed and assured than it was in his previous feature, "Bad Company." Before turning to writing, Benton had been an illustrator and art director at Esquire, and that experience seems to serve him well in "The Late Show," which is remarkable for non-studio sets decorated and detailed with unfailing smartness - Margo's apartment, Birdwell's hot merchandise warehouse, a blood-spattered suite with a trail of blood-spattered suite with a trail of blood leading toward a flowered bathtub curtain.

The outbursts of violence have the startling, incongruous quality that helped the first movie Robert Benton and David Newman wrote, immortalize "Bonnie & Clyde." The violence in spectacular, but it possesses the identical suddenness and sense of dismay.

By inventing a private eye of his own, Benton also gives the film a measure of integrity and originality missing from both "The Long Goodbye" and "FarewelI, My Lovely," where Chandler's detective hero, Philip Marlowe, was caught in a time warp, too young in one setting and too old in another. Instead of belaboring detective who happens to belong to the same eneration and reflect a similar outlook.

Ira's professional skill may be more satisfying because we can see how vulnerable his age and health make him. It's customary for the shamus to get abused and look for his opportunity to get even, but the reenactment of such ritual scenes in "The Late Show" seems to have an added impact and satisfaction. There's a special quality of heroism in Ira's suffering and touchness, derived from the belief that this case might really be his last.

"This town hasn't changed," Ira grumbles. "The same dames screwing up their lives in the same ways." Screwed-up Margo isn't beyond redemption, as her attraction to Ira testifies. She has the right idea even if she persists in expressing it preposterously: "We'd be a team You've got all this experience, and I'm this very on-top-of-it person. It's perfectly true that you're a slob and I'm a Virgo, but can't you see? It's the combination that makes things interesting."

I don't think Lily Tomlin's performance is as emotionally satisfying as Carney's. Margo is such a prattler that one can almost abstract her from the movie. Now and then one seems to be watching a Tomlin monologue rather than a character, but it's not an impression that lasts. Tomlin is mainly an ingratiating screwball original from the moment she first tries to wheedle Carney into looking for her kidnapped cat: "Maybe you should learn respect for other little creatures." It is, indeed, the combination that makes things interesting, and "The Late Show" seems to have flourished because of two fortunate combinations: Carney and Tomlin in front of the cameras and Benton and Altman behind them.