Two old folks in an Idaho family: a humane, effective story; Elba Play by Vaughn McBride. Directed by Tom Bullard

It is summertime in the 1960s in Elba, Idaho. Don and Flo have temporarily escaped from what Don calls ''the Flossie Patch Home for the Criminally Old.'' Although she had never before been behind the wheel of a car, Flo managed, with Don's instructions, to drive the stolen laundry van in which the getaway was made. News of the flight already has been broadcast over the Elba radio station. Having made it back to their modest ranch house, the elderly couple await the inevitable arrival of the sheriff to escort them back to Flossie's institutional care.

Meanwhile, playwright Vaughn McBride employs the time for a sympathetic consideration of the old folks' present plight and certain of the events leading up to it. To relate past and present, ''Elba'' introduces the couple's daughters: the selfish, flighty, and vulnerable Harley (Ann Wedgeworth), whose unwelcome husband lurks in the camper parked outside, and the more reliable Lete (Barbara Sohmers). The death of a third daughter while en route to Elba heightens the emotional crisis and leads to the play's sad but gallant resolution.

Following in the wake of other similar stage reflections on the cares of the aging and the caring for their needs, ''Elba'' conveys a relished appreciation for the comic saltiness of its elderly protagonists and a humane concern for their plight. The performance staged by Tom Bullard brings a needed sense of conviction to a script that seems at times to spring more directly from Mr. McBride's determination to cover all possible bases than from the specifics of the situation.

In his familiar technique, James Whitmore is both effective and affecting as the once-sturdy rancher whose mind now is alternately sharp and befuddled and who has a habit of addressing himself directly to a Life magazine cover portrait of Franklin D. Roosevelt mounted on the wall. Audra Lindley's Flo is sharp, protective, and maternal as the sprightly accomplice in this audacious but short-lived fling at freedom. Miss Wedgeworth and Miss Sohmers contribute believably to the Idaho family portrait, while Frank Hamilton adds local color as ''Young Roy'' Eames, who is sent out to catch a plump live chicken for dinner and instead returns with Kentucky Fried. Colonel Sanders would at least have been pleased.

''Elba'' has been designed with a nice feeling for its time and atmosphere by Kate Edmunds (set), Patricia McGourty (costumes), and Dennis Parichy (lighting).