THIS IS A FUNNY picture about Jews and Gentiles. Not a very original idea, but uniquely successful in this case, because director Elaine May and her daughter Jeannie Berlin work blithely yet subtly within the motif, transforming its simplemindedness into simplicity and its banality into unpretentiousness.

The bare bones of the story are familiar, and, once into it, almost predictable. Lenny (Charles Grodin), a nice Jewish guy, marries Lila (Jeannie), a nice Jewish girl, to the strains of that old piano favorite, "I'd like to buy the world a Coke..." On the trip from New York to Miami for their honeymoon, Lila gradually reveals her clumsy, frumpy self (and the wide range of Berlin's comic talents), to the growing dismay of her husband. She proudly thrusts her bare breasts at him in the car, nearly causing an accident. She chews gum loudly, eats candy in bed, and constantly chatters about what their life will be like after 50 years of marriage. Lila suffers from a fatal form of Midas disease--everything she touches turns to caricature. She has the knack of making a word like "grouch," her favorite epithet for the uncooperative Lenny, grate on the nerves like fingernails down a blackboard.

Lila's mistake is to burn herself under the Miami sun to the point of immobilization, leaving Lenny free to explore the delights and terrors of the Gentile world. He falls madly in love with Kelly Corcoran (Cybill Shepherd), a nymphet whose sleek beauty and poise draw him all the way to her Minnesota home--and to a hurried divorce from Lila. Lenny's main obstacle to winning Kelly's hand is her father, terrifying portrayed by Eddie Albert, a sort of Old Testament God who threatens to wreak an "ass kicking" on Lenny if he doesn't leave the schikse alone and return to his own kind. But Lenny is persistent, and he finally weds his bride, managing a triumphant grin at the Protestant minister.

May's direction is characteristically light and bouncy, a necessity given the heavy-handed theme, but she does not skim over elements of sometimes tragic seriousness. Underlying Lila and Lenny's relationship, as May develops it, is an escalating dynamic of disintegration which lends the humor and fun a keen psychological edge. Lila, for all her obtuseness, senses from the start Lenny's reserve--but he refuses to tell her anything is wrong, to bring it all out in the open, since his trust and confidence in her has been broken. She frantically tries all the harder to communicate and to please, but her pathetic awkwardness only increases his alienation. They drift further and further apart, till the final separation.

Their last evening together is the climax to this tragicomedy of errors. Over sumptuous dinner, Lenny tries to break the news of his decision to divorce her. When he finally makes it clear, Lila sobbingly responds in character: "Lenny... I think I have to throw up." She finally collapses into his arms, and he tries to ward off the poignancy of the moment with empty consolations: "When two people share a common tragedy..." But it defeats him; at the end of the scene, Lila cries on with the nobility of the victim, as he falls into guilty silence.

Lila's departure is in one respect a relief to an audience torn between laughing at her buffoonery and crying at her deep anguish. But without her, the movie loses its humanness, and is quickly trivialized into an exchange of bon mots. Neither Lenny nor Kelly has a heart to be broken, and that is precisely the problem. Cybill Shepherd in particular lacks the range of acting emotion necessary to sustain the human relationships at anything more than a superficial level; a serious flaw in the latter part of the film is her inability to warm up to the very man with whom she is supposedly in love. Her self-consciously coquettish treatment of everyone from the maid to her father is proof that her virtually identical performance in The Last Picture Show was no accident, but a fact of her nature.

AS THE HARD-HEARTEDNESS of the characters increasingly manifests itself, strange scenes unfold. Lenny is now so possessed by his ambition to capture Kelly that he'll ruthlessly resort to the most outrageous sycophancy. At the Corcoran dinner table, for instance, he comments on the meal in tones reminiscent of Nixon courting the farm vote: "Now this is honest food... There's no lying in that beef... no insincerity in those potatoes ... no deceit in that cauliflower."

Later on, the couple rendezvous in a mountain cabin, where Kelly devises a "game" involving the removal of all their clothes, the object of which is to get as close to each other as possible and yet not touch. Lenny passes the test, which scholars have discovered was sometimes required of clerical novices in the Middle Ages, who had to spend a night in the same bed with an unclad nun and resist all temptations in order to prove their piety. The point of such an exercise in this film, aside from the injection of cheap suspense into a sagging plot, is as yet unknown.

MAY'S EXTRAORDINARY EYE for comic, human detail prevents all but a few such bald moments from materializing. On one occasion, the physical possibility of holding hands comfortably while lying side by side is used to subtly underscore Lila and Lenny's uneasy relationship. To emphasize Lenny's guilt and embarrassment during the break-up scene, the camera matches Lila's cries or his frantic whispers with closeups of disapproving restaurant customers within earshot. Lenny's frigid welcome in Protestant Minnesota is highlighted by a monotonous radio voice droning the sub-zero weather report, which seems to follow him everywhere.

Despite such nuances as May and her cast provide, the artistry of the film is ultimately limited by its very theme. Bruce Jay Friedman's short story, on which the movie is based, has lost its comic edge in this post-Portonoy era, so much so that the ethnic overtones in the film are often annoying. That this picture, technically excellent in so many ways, should bump up against thematic cliches which become embarrassments and irritants, is an indication that the arts and entertainment people have milked dry yet another erstwhile sacred cow.