LOS ANGELES (AP) _ The American Civil Liberties Union filed suit Monday against Big Brothers of Los Angeles, alleging discrimination against homosexual or bisexual men who wish to participate in the program.

The Superior Court lawsuit seeks $250, a minimum required by state law, and asks that Big Brothers be ordered to stop excluding men who are homosexual from the program, which pairs ''substitute fathers'' with boys to provide support and insight on growing up.

ACLU attorney Susan McGreivy, representing bisexual plaintiff Richard Stanley in the suit, said that while homosexual men should be allowed to serve, the ACLU supports the right of mothers or guardians to reject them.

Richard Kline, president of Big Brothers of Los Angeles, defended the group's procedures.

''Since a Big Brother is looked to for information from everything from school to sex, we don't believe it is appropriate to match a homosexual man or bisexual man with a fatherless boy,'' said Kline.

''It is not our function to expose a boy to an alternative lifestyle at a time when he is struggling with his own identity.''

Kline said homosexual men were not necessarily bad role models for boys but that Big Brothers believes homosexuals ''are not the best possible role models.''

Kline said he fears the suit may prompt other groups to challenge Big Brothers' stringent screening procedures, which check for criminal history and many references in addition to sexuality.

Stanley, 35, a car salesman from Santa Monica, said he was told he was rejected because he admitted during a screening interview that he was bisexual.

''I see no reason why I could not be an appropriate role model,'' said Stanley, who was raised by his mother in a single-parent household. ''I always wished I had (a Big Brother).''

Ms. McGreivy, noting that the suit seeks minimal damages, said the ACLU doesn't wish to harm Big Brothers because the group's service to society is great.

She said sexual preferences are already formed by age 6 when a child becomes eligible for a Big Brother.

Ms. McGreivy said she's a homosexual parent herself and that her two sons, ages 17 and 19, are both heterosexuals.

''They're just regular little guys. They both have girlfriends - well, one has a girlfriend and the other's heart is hoping,'' she said.

If somebody were to lie about their sexual preference, that would be legitimate grounds to reject them, she said.

Big Brothers of Los Angeles is a chapter of the Philadelphia-based private nonprofit volunteer group.