Small experimental theater groups provide a lively undercurrent to the theater life of a big city. For theatergoers with a sense of adventure and curiosity, the risk of disappointment is worth the chance of discovery of a good production or a cameo performance.

Relying more on tenacity, energy and dedication than money, these troupes persist despite playing in makeshift theaters before small audiences that sometimes don't number a score.

Such a group is the Fine Line Theatre Company, an outgrowth of the Earth Onion Women's Theater. The Fine Line group is presenting "The Effect of Gamma Rays on Man-in-the-Moon Marigolds" through next Thursday on the loft stage of the Washington Projects for the Arts.

The play brought Paul Zindel the Pulitzer Prize for drama in 1971.

Zindel's drama is a bittersweet story about dreams that go sour and the half-lives of people as well as that of Cobalt 60. Some manage to keep a part of their dreams and survive.

"I almost forget everything I was supposed to be . . . What's left for me?" asks Beatrice, a woman made bitter by disappointments of life.

Beatrice lives with her two teen-age daughters and cares for a senile woman to help pay the bills.

The elder daughter, Ruth, ranges from vulnerability to vindictiveness in her behavior. Tillie, her other daughter, is stocky, bright and at the ugly stage in life. She is awed by the wonder of the world of atoms and is conducting an experiment on the effect of radiation on marigolds.

When Tillie becomes a finalist in the science fair at her high school, Beatrice has a chance "to feel a little bit proud about something." But Ruth destroys the moment by recalling an ugly nickname from Beatrice's own high-school days.

Despite the disastrous evening, Tillie is the science-fair winner and still has her wonder of the world of atoms. She will survive with that.

Jane LeGrand, the director of the Fine Line troupe, is excellent as Beatrice. She captures both the mother's bitter side and the sadness of a woman of lost dreams. The most striking scene comes when this wisecracking hard mother shows tenderness toward Ruth, who has awakened from a nightmare, and the two talk as a flashlight casts light and shadows.

For some reason, Ruth, played competently by Judith Z. Miller, is made up to look like a 14th Street tart. This misfortune also happens to Lianne Cohn, who exudes the wonder of Tillie but can't quite overcome the image of the goody-good girl with a costume of knee-high socks, a velvet dress, and patent-leather pumps.

Theresa Aceves is strikingly good in her small role as another finalist in the science fair.

Both the pet rabbit and Lee Grant, who plays the disabled stroke victim, have no lines but perform admirably.

"This is a production on a $2.49 budget," said Ellie Chamberlain, the founder of the Shakespeare Summer Festival, who directed the production.

The Fine Line production is good value. The shows are at 8 p.m. through Thursday (dark on Monday) at the Washington Projects for the Arts, 1227 G St. NW.