New Jean Kerr comedy is a tossed summer salad; Lunch Hour Starring Gilda RAdner, Sam Waterston. Comedy by Jean Kerr.Directed by Mike Nichols.

"Lunch Hour" is light summer fare in a cold season. The new entertainment at the Ethel Barrymore Theater turns out to be a tossed salad of a comedy, expertly prepared by Jean kerr, a lady who knows all about crisp humor and the light touch. The posh expanses of Oliver Smith's Southampton beach-house setting provide a natural locale for Mrs. Kerr's discourse on modern marriage and the infidelities that too often afflict it in the upper-bracket milieu of contemporary worldlings. But in this case morality triumphs.

The contretemps begins when Oliver (Sam Waterston), a Park Avenue marriage counselor and pop author, learned that his heretofore faithful wife, Nora (Susan Kellerman) is having an affair. Bearer of the distressing news is Carrie (Gilda Radner), the wife of Nora's lover. Drawn together by their mutual plight, Oliver and Carrie devise various schemes with which to break up the liaisons threatening their respective marriages. They even consider -- and ultimately reject -- the notion of fighting fire with fire.

How "Lunch Hour" untangles these marital snarls may be left for the final twist of Mrs. Kerr's slight plot. Meanwhile, she and Miss Radner are beguiling the spectator with Carrie's winning and sometimes touching ways. Carrie fetchingly combines literal-minded naivete and practical common sense. A one-time fat girl who has dieted down to a sylph, she suspects that her loss of weight may have had something to do with her loss of husband. Carrie's butter-fingered propensity for spilling and breakage are an index of her insecurities.

Miss Radner, who rose to stardom on TV's "Saturday Night Live," takes her Broadway acting debut in spunky stride. She has long since mastered the timing and sharp observation demanded for review sketches. Now she proves herself able to sustain character in a dramatic situation.

Co-star San Waterston gives a fine comic performance as the would-be detached professional who finds the little match girl "not entirely unappealing." Mrs. Kerr understands the appeal. Indeed she regards the frailties and foibles of all her affluent Long Island summer folk with a benign amuement that the audience shares. Besides those already mentioned, the attractive cast urbanely directed by Mike Nichols includes David Rasche as Carrie's super-rich husband and Max Wright as the obliging downstairs neighbor who owns the beach house where "Lunch Hour" runs its breezy course.