"Best Boy," opening today at the K-B Cerberus and Georgetown Square, is one of the most touching and revealing real-life stories a filmmaker has even been privileged and astute enough to document.

The filmmaker is Ira Wohl, a 35 year-old New Yorker who tried to help his mentally retarded cousin and then had the inspired afterthought of recording the consequences as they emerged over a three-year period.

It's obvious that "Best Boy" could not have evolved into the perhaps unprecedented human testament it became without a special set of circumstances different from most fictional movies. Wohl was on itimate terms with his subjects -- his aging aunt and uncle, Pearl and Max Wohl, and their mentally retarded son, Philly, aged 50 when the chronicle begins -- and he could wait for their story to be resolved. This special relationship allowed Wohl access to rich emotional material, but in return he respected the trust placed in him and redeemed it with a loving, haunting family portrait.

Although its one of the few documentary features likely to attract a large theatrical public -- especially after winning an Academy Award -- "Best Boy" may have to overcome certain forms of audience resistance. To deal with the most potentially stubborn misconception, "Best Boy" is not a documentary about mental retardation. In the extraordinary intimate, congenial atmosphere of the film the intellectual limitations Philly demonstrates help to define and differentiate him, but they never encourage one to feel patronizingly sorry for or superior to him. He possesses too much vitality and individuality.

Moreover, one begins to feel like a member of the family -- both because Wohl is recording episodes in the on-going life of his cousin, aunt and uncle and because the Wohls prove to be exceptional camera subjects. Philly and his mother, Pearl, in particular reveal affinities for the camera that are at once mysterious and devastating.

The events in the film were prompted by Ira Wohl's well-meaning suggestion that it might be advisable to seek institutional support for Philly, since his aunt and uncle were nearing 80 and Max's health was precarious. Evidently retarded from birth, Philly was the youngest of .BEST, From E1> three children. His older sister, Frances, appears in the film. An older brother, Henry, called "Honey" within the family, died a few years ago.

With the exception of a brief period in adolescence when he was enrolled in an institution (recalled as hateful by both father and son), Philly had lived with his parents, growing to middle age in their middle-class homes in Brooklyn and Queens. He was protected and cared for by loving, diligent parents resigned to bearing a puzzling, humiliating yet also undeniably affectionate human burden. Formal education was neglected or unavailable, and the idea took hold that Philly had never advanced beyond a mental age of about 5.

Wohl began his film shortly after making contact with a private organization called the Association to Help Retarded Children. Following a series of tests and interviews, Philly was accepted at a day school in Manhattan surpervised by AHRC and thrived there. After Max's death, an opening became available for Philly in a new group residence for retarded adults run be AHRC. The widowed Pearl resigned herself to this belated, final separation. At age 53, Philly Wohl was finally able to move away from home and join his peers.

The satisfaction one derives from Phily's progress toward a measure of adult dignity and independence is tempered by the realization of what a decisive emotional blow his mother endures. Pearl emerges as one of the greatest maternal figures in the history of the movies, an image of pathos as emotionally complex and suggestive in its spontaneous, unrehearsed way as Katharine Hepburn's embodiment of the tragic Mary Tyrone in the film version of "Long Day's Journey into Night."

Of course, the Wohl household is not a hotbed of recrimination in the tradition of O'Neil's unhappy Tyrones. On the contrary, the pathos of "Best Boy" is underscored by comic motifs, an authentic expression of the idiosyncracies of Philly, Pearl and Max (three of the most unforgetable characters you ever met) but also a result of one's recognition that a quintessential family story is being played out with stunning ironic twists.

An idiomatic Jewish humor links father, mother and son in oddly distinctive, compatible ways.

There's an irresistible antic streak in Philly, evident in sudden exclamations like "i'm gonna tanz the hotchacha like in the movies!" or solicitous offerings like "Sit down, Max -- relax your bones a bit, you'll feel better" or his spontaneous vocalizing on "happy Birthday." "As Time Goes By." Anniversary Waltz" and other favorites. b

Phill'ys lusty, garbled singing is also a pervasive reminder of the joyful, self-expressive urges bottled up in his sweet dispositon and slightly miswired mentality. It's obvious that he needs to break out, to assert himself as far as prudent supervision can permit. One hears the urgent, childish exasperation in his "Yeah, yeah, yeah, I understand I understand" when being reminded of things repeatedly.

Philly's prolonged role as the baby of the family has also created tensions between Pearl and Max that are both witnessed and inferred. One of the most heartening scenes in the film reaffirms their emotional bond with breathtaking impact. Returning from the hospital, frail and nearly blind, the scowling, taciturn husband turns to his wife and says, "You know I missed you?" Overcome with his unexpected demonstration of affection, Pearl begins to weep happily and kisses the top of his bald head.

"What are you crying about?" Max growls.

"I don't know," Pearl replies, "I'm just crying."

This kind of eloquence is achieved time and again in "Best Boy." Pearl shows a phenomenal ability to say just the right suggestive or summarizing thing at exactly the right moment. For example, no invented dialogue could improve on the remark that slips out when she's visiting the residence and sees that there's a spare room: "Maybe I should move in. It wouldn't be a bad idea if I moved in here." Or the confidences she shares with her nephew's camera and movie-goers for generations to come: I'm lost without him. I can't help it. Fiftynine years is a long time with one man . . . I just feel depressed, don't mind me. You know how you feel when you love a man? For always? That's how it is . . . My life has ended. wYou know, my life has ended."