Not on the scale of a tragi-comedy, "Benefit of a Doubt," a new play by Edward Clinton at the Folger Theater Group, is a funny-sad.

Using a harmless family-comedy routine to show a family with serious problems, it deals with the theme of tolerance, and its limits.

The family is a simple and well-meaning one, living fairly comfortably in West Virgina, where the father is a miner. They are inching toward their dream of having a hamburger-stand franchise. Their chief problem is to make a 14-year-old retarded daughter feel accepted in a world that stares at her; next is the sour presence of an increasingly senile live-in grandmother, who is a model of religious intolerance.

The strain creates other problems. At the play's opening, the wife has just pulled herself together, having attempted suicide and then run away following a stillbirth. The husband generally manages to do his part, but has the occasional outlet of making a gently hostile gesture, such as deliberately anticipating the daughter when she wants to surprise him by counting to 10.

But they are carrying on, with imperfect amounts of understanding and patience, trying to live up to their feelings of love and sense of duty. It's a milieu in which a major act of kindness is telling a nursing home-bound father that America won the war in Vietnam, and a grand gesture is deciding to go to Disneyland, instead of the closer Disney World, because it's the "real," original park.

This is a play with rough spots. The wife's escape, it turns out, was to indulge herself at art museums with a non-sexual friend who was "crazy about painting," as if the playwright were embarrassed about the family's working-class tastes. When she becomes pregnant again, the question about extending the benefit of the doubt to another possibly retarded child is not the issue; it is the fact that she has been unable to carry other pregnancies successfully, a medical, rather than moral, issue.

But Carol Kane puts so much "special" sweet innocence and "special" repetitive wearisomeness into the retarded daughter as to establish with enormous poignancy the position of those who love her. A scene with Ray Aranha, as an alcoholic who befriends her, is poetic.

It is jarring that Elizabeth Council, as the grandmother, and Mikel Lambert, as a sister, play for unsubtle comedy. But the parents, played by Geraldine Court and Stephen Mendillo, are a unifying force as they richochet between buoyancy and despair.

BENEFIT OF A DOUBT - At the Folger through May 13.