Kenneth Frankel, 56, Director Won Obie for Broadway Show

Kenneth Frankel, a prolific stage director who won an Obie award for his production of Simon Gray's ''Quartermaine's Terms'' and staged the premiere of Mark Medoff's ''When You Comin' Back, Red Ryder?,'' died last Thursday at his home in Los Angeles. He was 56.

The cause was a brain tumor, said his wife, Donna Isaacson.

As a director on Broadway, Off Broadway and in regional theater and as an associate artistic director of the Long Wharf Theater in New Haven, Mr. Frankel staged a diverse group of more than 40 plays, but he specialized in intimate productions of naturalistic contemporary drama, what he once termed ''humanist theater,'' plays about ''characters and relationships, things that happen to people,'' as opposed to ''politics, didacticism or spectacle.''

With Mr. Gray's ''Quartermaine's Terms'' and ''The Common Pursuit,'' James Saunders's ''Bodies'' and plays by Harold Pinter and Arnold Wesker, he proved to be equally adept at staging modern English works for American audiences. One of his hallmarks was bringing together American actors, and occasionally English ones as well, into an ensemble that equaled the plays' original London casts. Many of his productions moved from Long Wharf to Broadway and Off Broadway.

Mr. Frankel was born in Cleveland and graduated from Northwestern University. After working at the Arena Stage in Washington, he became an assistant to Tyrone Guthrie during the early years of the Guthrie Theater in Minneapolis.

He made his first breakthrough as director with his taut production of ''Red Ryder'' in 1973 at the Circle Repertory Company. Kevin Conway starred in the Medoff play as a swaggering con man who awakens a sleepy New Mexico diner. Mr. Frankel later directed Mr. Medoff's ''Hands of Its Enemy'' at the Manhattan Theater Club.

For six years, he served as artistic director of the Dallas Shakespeare Festival. In the late 1970's, he staged J. M. Barrie's ''What Every Woman Knows'' and Stewart Parker's ''Spokesong'' at Long Wharf. Later as an associate director there, he did as many as four plays a year, moving from J. B. Priestley's ''When We Are Married'' (starring Donal Donnelly) to ''The Heiress'' (with Richard Kiley), while also doing musicals (''Pal Joey'') and new plays (Leonora Thuna's ''Fugue'').

His 1983 production of ''Quartermaine's Terms'' (with Remak Ramsay, Caroline Lagervelt and Kelsey Grammer) was a long-running Off Broadway success. In 1984, he directed a revival of Mr. Pinter's ''Old Times'' with Anthony Hopkins, Jane Alexander and Marsha Mason, and in 1986, ''The Common Pursuit,'' with Nathan Lane.

In addition to his wife, he is survived by their daughter, Hanna, of Los Angeles, and by his father, Elmer, his brothers, Douglas and Thomas, and his sister, Alice Haas, all of Cleveland.

In recent years, Mr. Frankel made the transition from stage to television, directing episodes of ''The Days and Nights of Molly Dodd'' and ''Doogie Howser, M.D.'' But he remained devoted to regional and institutional theater.

''That's my home, and that's my conscious choice,'' he said about his years at Long Wharf. ''Rather than go with the winds of fate as a freelance director, I want to be able to look back on my life and say, 'This is where the work was done.' ''

The director won an Obie award for his work on the off-Broadway production of Simon Gray's "Quartermaine's Terms" at Playhouse 91 in 1984.

Frankel first made his mark in New York theater in 1973, when he directed "When You Comin' Back, Red Ryder?" Among his other 40 stage productions was Harold Pinter's "Old Times," starring Anthony Hopkins, Marsha Mason and Jane Alexander.

Predominantly based at the Long Wharf Theatre in New Haven, Conn., Frankel directed plays at regional theaters around the country including the Pasadena Playhouse. He was artistic director of the Dallas Shakespeare Festival for six years and served on the National Endowment for the Arts theater panel.

Although the Long Wharf sent several plays on to Broadway, Frankel never saw that as a major goal.

"We don't think in those terms, only whether the play is good for our theater and our audience and whether we're excited about doing it," he said in 1984. "Producers may be coming to see it, but we can't let that blur our intentions. Any more is just frosting on the cake."

His own reputation enabled him to move permanently to Broadway and off-Broadway, but he stuck with regional theater. "I want to be able to look back on my life," he once commented, "and say, 'This is where the work was done.' "

Frankel also did some directing for television, including "The Days and Nights of Molly Dodd," "Doogie Howser, M.D." and the 1992 television special "I Remember You."

Born in Ohio, he graduated from Northwestern University and earned the Tyrone Guthrie Fellowship to work with Guthrie at the Guthrie Theatre in Minneapolis.

In addition to his wife, Frankel is survived by his daughter, Hanna, of Los Angeles; and by his father and stepmother, Elmer and Peggy; two brothers, Douglas and Thomas; and a sister, Alice Haas, all of Cleveland.

The family has asked that memorial donations be made to the Manhattan Theatre Co., 113 43rd St. 8th Floor, New York, NY 10036.

A funeral service is scheduled for 1 p.m. Tuesday at Temple Isaiah, 10345 W. Pico Blvd.