ABC begins an eighth season of "Afterschool Specials" today with "Which Mother Is Mine?," an admirable and absorbing drama about a beleaguered foster child, at 4:30 on Channel 7.

Melissa Sue Anderson (Mary on "Little House on the Prairie") plays a 16-year-old girl whose mother gave her up at the age of 6 and who has spent the past six years living happily in a foster home. Then, from out of an alcoholic past, the mother, cured, returns to reclaim her daughter.

Ensuing trauma is thoughtfully and realistically dramatized by writer Durrell Royce Crays and by directer Arthur Siedelman, whose only noticeable lapse is a little sloppy cross-cutting early on.

The girl's dilemma is compounded when she meets the long-absent mom and, instead of hating her, recognizes her mother's own victimization and vulnerability. A court is to rule on the matter in such clinical terms as "petition of abandonment," and the girl must decide whether she wants to stay with her foster family or return to the mother she only really knows from nightmares that flash back to childhood.

A tough subject is handled without snap solutions or sermonizing. The girl learns from her boyfriend (dependable Eric Scott of "The Waltons") that her fate isn't just a matter for the disposition of others; sometimes you have to "fight for what you want" even though "somebody gets hurt" in the process.

Afterschool Special Land continues to be a creative off-Broadway compared to the gross and glossy on-Broadway of prime time. Actors like Marion Ross, of "Happy Days," prove they really can handle something beyond reaction shots (though there are still too many of her here); Anderson's performance is touchingly detailed, and Virginia Capers is the soul of compassionate authority as Judge Atherton.

Unfortunately, a scrupulously humanist tone is repeatedly betrayed by raucous commercials, including several appearances of the tediously demented Ronald McDonald and that Spray 'n Wash ad in which a woman gets doused with her own dinner, and in slo-mo, yet. And the show's opening is marred with the ABC promotion department's sleazy idea of a tease, one that makes "Mother" sound just like what it serves as an antidote to, TV's usual soapy scam.

The ABC on-air promotion department is the kind of thing you wouldn't want to meet in a dark alley.