Ignoring the contribution Buster Keaton made to American culture, and to the world's disposition, would be like overlooking Mark Twain. Maybe it would be worse. In film clip after film clip on a PBS special tonight, what comes through in Keaton's work is brilliance, greatness, even genius.

"Buster Keaton: A Hard Act to Follow," the latest entry in the extraordinary "American Masters" series, traces Keaton's work and life with excerpts from his great silent comedies, interviews with associates, and comments Keaton made to a Canadian interviewer in 1964.

There are more priceless gems contained in these three hours of television than there are displayed at Tiffany's and Harry Winston's put together. But "Keaton" is also a terrific dramatic story, the kind so incredible that it could only be lived in Hollywood, yet probably not invented there.

The three-part program has been curiously slotted; Parts 1 and 2 tonight at 9 and Part 3 next Wednesday at 9 on Channels 26 and 32; PBS bureaucrats are so inscrutable. Perhaps, though, public TV will schedule some of the old Keaton two-reelers and features in full-length versions.

Time turns craft into art. Keaton's meticulously fashioned visual gags and wizardly tumbles now seem more than inspired tomfoolery. Surely there's some effortless human-condition stuff lurking within his continuing episodic saga of a doleful, wan-eyed everyman mercilessly hounded by circumstance. When hundreds of angry cops weren't chasing him, hundreds of husband-hunting gold diggers were. A house could fall on him, and it did; with heroic resilience, he pressed on.

In his life off the screen, Keaton was not so tenacious. Born Joseph Keaton Jr. in 1895, he was renamed "Buster" for a fall he took down the stairs at a tender age -- the first of hundreds to come. To get around child labor laws with the family vaudeville act, his parents passed Buster off as a midget. He first crawled onstage at the age of 9 months.

Though he made a fortune as a silent screen star, Keaton lost it after the talkies arrived, not because he couldn't translate to the new medium but, says the program, because he came under the domain of Metro-Goldwyn-Mayer and its oppressive, at least to comics, factory system (Laurel and Hardy and the Marx Brothers suffered under it as well).

At 37, Keaton was fired from MGM by Louis B. Mayer. He was divorced, depressed, impoverished and alcoholic. Not until his rediscovery in the '50s, and the unearthing of long-lost Keaton films (James Mason found some in the basement of a house Keaton once owned), did he receive his proper share of acclaim. He had a harrowing, poignant, roller-coaster life, but when Hollywood made "The Buster Keaton Story" in 1957, it concocted a new one for him. Maybe the truth seemed too cruel.

Film historians Kevin Brownlow and David Gill, who previously triumphed with "The Unknown Chaplin" for this series, wrote and produced the Keaton show for Britain's Thames Television. Everything about it is splendid, except perhaps the choice for narrator: director Lindsay Anderson, whose snooty British accent is at first off-putting and decidedly un-American.

But the producers show a knack for using Keaton's films to comment on his life and vice versa, without ever getting cutesy about it. When Keaton's alcoholism is noted, and a crony recalls, "Buster drank out of loneliness," the fact is counterpointed with slapsticky scenes from a 1933 Keaton romp called "What! No Beer?"

"As his character was tested in his films, so it was tested in life," says narrator Anderson, not stretching the point.

Ironically or not, the filmmakers of the silent era who loftily imagined they were preserving immortal works of literature for the edification of the masses produced work that, for the most part, hasn't lasted. It's dead. But Keaton, who worked unpretentiously and tirelessly to concoct funny gags and riotous crowd-pleasers, can now be seen as much more the artist. He understood film.

"I always wanted an audience to outguess me," Keaton says looking back, "and then I'd double-cross 'em." There is ample evidence of his doing that time and again, as in a dazzling fake-out involving one car and two trains from the 1931 "Parlor, Bedroom and Bath," included in Part 3.

"Keaton" is full of the juiciest kind of Hollywood lore. The star's earliest on-screen partner was Roscoe (Fatty) Arbuckle; the tragedy of Arbuckle's scandalous demise (even though he was acquitted of a murder charge, his career was over) served to liberate Keaton to be his own man both on the screen and behind it.

The freshness and invention of his finest films, "The General" and "Steamboat Bill, Jr.," are still unmistakable, and the prints used and restored for "Keaton" are, for the most part, vivid and luminous, unlike the tattered and faded Keaton shorts that show up as filler on the Showtime cable service.

Among those interviewed for the documentary are Keaton's widow Eleanor, whose reminiscence is disarmingly affectionate; Donald O'Connor, who played Keaton on the screen and values the memory of his counsel; actor James Karen (so hilarious in "Revenge of the Living Dead") who appeared with Keaton in his twilight project "Film," written by Samuel Beckett in 1965; and Allen Funt, for whom Keaton made an uproarious "Candid Camera" appearance as a fumbling bumbling diner at a luncheonette.

Sadly, only a snippet of that is included. Apparently rights could not be obtained to any scenes from "Limelight," the only screen appearance Keaton made with Charles Chaplin. Nor do we get to see the cut Keaton scenes shot for Stanley Kramer's "It's a Mad, Mad, Mad, Mad World," though the program includes such rarities as a '50s Keaton gasoline commercial and re-creations of his vaudeville routines done for "The Ken Murray Show."

There never seems to be enough of any of the Keaton films in this documentary but that's all right; it whets the appetite and sets the stage for a full-scale revival. If only PBS stations would stop purchasing so much disposable commercial junk (old Lawrence Welk and Walt Disney network discards, for instance) and show more enterprise about unearthing American treasures.

In every detail, "Keaton" is a work of care and distinction. That includes the piquant musical score by Carl Davis. When Keaton goes to MGM, Davis sneaks in the elegantly elegiac theme he wrote for another Thames series on the silent years, "Hollywood," a show so good it should be aired annually.

Etched into Keaton's celebrated granite face, one can see, in the later footage, the toll taken by years of pain and rejection, even after he'd become a world star. "Keaton" makes the case better than it is usually made that the movies lost something irrevocable when they learned to talk. In the Canadian interview, Keaton complains that at MGM, humor was measured in words, not actions, that he longed to go "six, seven, eight, nine minutes" without the intrusion of dialogue.

Yet as he refused to grovel for an audience's sympathy in movies, he seemed free of resentment or regret in life. "It was a lot of work," he says simply, "but it was a lot of fun." This essay on Keaton is more than a fascinating evocation of early Hollywood; it's a moving account of brave pioneering. Whatever cinematic subject Brownlow and Gill tackle next, "A Hard Act to Follow" is certain to prove just that.

'Jerome Kern'

Johnny Mathis, Roberta Flack and a juvenile dancing duo are the showstoppers on tonight's "In Performance at the White House" tribute to melodist Jerome Kern, but the evening owes much to one who isn't there: Nancy Reagan, who missed this blithe musicale because she was hospitalized for surgery when it was taped in October.

"I miss her a lot," says emcee Marvin Hamlisch near the top of the special, which airs at 8 on Channel 26, the producing station. He dedicates the concert, which features "the most romantic ballads ever written," to her.

Those ballads include "Smoke Gets in Your Eyes" played by Doc Severinsen; "Long Ago and Far Away" sung by Barbara Cook; "Dearly Beloved" and "Yesterdays" sung by Mathis; and a heavenly medley of "Bill," "They Didn't Believe Me" and "Can't Help Lovin' That Man" by Flack.

Cook's voice sounds thinner and less expressive than usual, and Severinsen looks as though he hasn't the foggiest notion where he is. Flack is her reliably wonderful self, but Mathis is the biggest surprise. After a slight wobble at the start of "Dearly Beloved," he handles the Kern tunes with consummate grace.

"In Performance" was executive-produced by John Musilli and directed by David Deutsch. As with the previous three installments, they manage to maintain dignity without becoming stodgy.

Eight lyricists are represented on the program. All the songs are standards, and one may wish the producers had included a novelty or two, as Joan Morris did on her sublime Kern album. But what's there certainly supports President Reagan's closing remark that Kern and the musical theater he helped perfect are "distinctly and joyously American." So is this happy hour.

'Lyle, Lyle'

"Operator, operator, this is an emergency. There's a crocodile in our bathtub."

But not just any old crocodile. Not even any old bathtub. This is Lyle, Lyle the Crocodile, reptilian hero of "Lyle, Lyle Crocodile: The Musical," a totally charming animated half hour premiering on Home Box Office at 7:30 this evening (and being repeated Nov. 23, 26 and 29).

Based on the long-popular children's book "The House on East 88th St." by Bernard Waber, "Lyle" tells of an affable creature who is not averse to drying dishes or helping little Joshua with his homework but who leaves East 88th Street for a spell after becoming famous for dancing in a parade. "Leaping Lizard!" headlines the Daily News.

Lyle's original owner, Hector P. Valenti (presumably no relation to Jack), wants him back, now that Lyle can earn his keep -- a keep that includes plenty of scrumptious Turkish caviar. Onstage, in Paris, Lyle proves infectiously heartbroken, and soon the audience is awash in tears not entirely crocodilian.

Engaging songs by Charles Strouse and Tony Randall's narration help the story along, and producer-director Michael Sporn spins it winningly. The animation is storybook-naif splashed with brash pastels.

Lyle himself is a poem in green. Does he get back to East 88th Street and its happily-ever-after existence? Well there's some serious kite-flying to do, so he'd better.