The Studio Theatre announced last week its 1986-87 subscription season, which will consist of six plays (up from the usual five), including British playwright John Byrne's "Slab Boys Trilogy," three plays that will rotate in repertory from September through November.

The first play, "Slab Boys," premiered in New York in 1983 and starred Sean Penn and Kevin Bacon. The comedy takes place in 1957 in the paint-grinding room of a rug-dyeing factory and tells the story of three boys who grind the pigment on marble slabs. Phil, Spanky and Hector are dreamers who plot hilarious ways of outsmarting their boss and fulfilling their fantasies, including how to win the affections of Lucille, the factory sketcher.

The second and third plays of the trilogy will receive their U.S. premieres at the Studio. "Cutting the Rug" finds the slab boys at their factory's annual dance and "Still Life" reunites the characters at the funeral of one of their own.

"It's a big risk because it's the whole fall," said Studio Artistic Director Joy Zinoman of the trilogy, which she will direct. "Are people willing to come to the theater three times?"

Zinoman called the trilogy an "event" and said, "We're trying to take advantage of the kind of energy in the old-fashioned serials, where [the audience] cares enough about the [characters] to find out what happens to them."

Opening in January will be Lanford Wilson's "Lemon Sky," a play about a young man who returns to his father's house to find a new stepmother, two half-brothers and a couple of teen-age foster children.

A revival of John Osborne's "The Entertainer" -- the story of aging British music hall performer Archie Rice -- will follow in March.

Rounding out the season in May and June will be Eve Merriam's "The Club," a tantalizing musical that takes place in a stuffy all-male club at the turn of the century and ends with a shocking revelation.

In addition to Studio's expanded six-play season, the theater will present a special holiday performance of Dylan Thomas' "Under Milk Wood" and "A Child's Christmas in Wales" in December.

Single ticket prices for next season will be held at their current level -- $11.50 to $13.50.

According to Zinoman, the Studio, winner of three Helen Hayes Awards this year, is having its most financially successful season in its eight-year history. Its current production, "Ma Rainey's Black Bottom," has been extended through July.

The Studio is currently renegotiating its small professional theater contract with Actors Equity, which allows it to employ a certain number of Equity actors, and was recently granted a constituent membership in the Theatre Communications Group, Zinoman said. On the Boards

Franca Rame performs her four-part monologue "Tutta Casa, Letto E Chiesa" (It's All Bed, Board and Church) beginning tonight in the Kennedy Center's Free Theater . . . Neil Simon's "Biloxi Blues," winner of the 1985 Tony Award for Best Play, opens tomorrow at the National Theatre . . . Horizons Theatre kicks off its New Voices '86 Festival tomorrow with "This Is the House," a play that explores the controversial topic of wife abuse and written and performed by the Human Bridge Theatre's Rebecca Rice and Janet Stanford . . . Julie Jensen's "Stray Dogs," winner of Arena Stage's Foundation of the Dramatists Guild/CBS New Plays Contest, begins previews Thursday in the Old Vat Room . . . The Washington Theatre Wing's production of "Very Truly Yours, M.L. -- A Visit With Mary Lincoln" has been extended through Sunday at the Black Box Theatre . . . Source Theatre performs "Agnes of God" Thursday through Sunday at the Castle Arts Center in Hyattsville as part of a program to bring Washington theater groups into the Maryland suburban area during the summer.