With more than 50 books to his credit, Howard Fast is definitely a pro -- and as a pro, it is fitting that he should be his own most effective critic.

A good one-sentence criticsm of "The Establishment" comes about five-sixths of the way through, when Barbara Lavette Cohen interrupts an intense conversation with her half-brother Joe to say, "If I read that in a book, I wouldn't believe it." Joe is a brilliant young doctor running a slum clinic in Los Angeles, whose wife has disrupted their marriage by unexpectedly becoming a movie star. What Barbara has trouble believing is that he refuses to sleep with his wife because she has become too rich and glamorous for his taste.

But then, Barbara has a lot of trouble believing things. She didn't really believe that she would go to prison for refusing to betray her old friends to the House Committee on Un-American Activities until, all of a sudden, it was happening. And she certainly didn't believe that her husband Sam would go off and get killed while he was helping the State of Israel to be born.

On the whole, Barbara does believe in herself, and that seems appropriate too, although she has done some fairly improbable things in this book and its predecessor, "Second Generation." For example, she puts her entire multimillion-dollar inheritance into a nonprofit foundation and sells her horse to get food for the soup kitchen where she had worked during the 1934 strike on the San Francisco docks. Barbara is relatively believable because she seems to be the one person, in the sprawling trilogy that began with "The Immigrants," with whom author Fast most readily identifies.

Except for Barbara, who brings the story to life whenever she comes on the scene. "The Establishment" is probably the least satisfying of the three books in which Fast has chronicled the ups and downs of the Lavette clan through half a century. A key to Fast's discomfort in this volume may be found in its title, which the author finds necessary to annotate twice and to illustrate in rather dry detail.

First we have Barbara's mother explaining to her the varieties of rich people. There is, she says, "a line of separation between fairly affluent people, like myself, for example, and those who are so enormously rich and powerful that they control the state. The British call them the establishment . . . "

Then 40 pages later, for those who weren't paying attention the first time, the explanation is given to Barbara's brother, Tom, who is becoming a member of the establishment. It comes from his wife, Lucy, who has some claim to hereditary position in it. "By those who know it exists," Lucy tells her husband, "the club is called the establishment. That's a British term."

"I know. I have been there," Tom replies rather petulantly, but nothing in the way he handles the subject will convince the reader that Howard Fast has been there. From his writing, it is easy to believe that he has been an observer at a hearing of the Un-American Activities committee, at a trial in a federal court and at a federal prison -- perhaps even that he has been in a bargaining session where Israeli agents were buying munitions with which to defend their new country. From his casual descriptions, it is easy to believe that he has spent much time in San Francisco and loved the city as it deserves to be loved.

But the board rooms and the drawing rooms of the powerful, the settings in which they buy a congressman or take over an industrial empire, have a ring not quite of inaccuracy, but of hearsay. If you want to see Fast at his best, go back to "Second Generation" and enjoy the precise, muted eloquence of the pages describing the San Francisco dock strike of 1934. Here the author is in his element, writing about people and scenes he knows at first hand, and the effect is striking. In contrast, the scene in "The Establishment" where a group of establishmentarians begin grooming a sleazy Republican congressman from California for the 1952 vice-presidential nomination seems lifeless and warmed-over.

The action of "The Establishment" takes place mostly in 1948 and the years immediately after, and Fast seems to be constrained to fall back on abstractions such as "climate of fear" when he tries to describe the distinctive flavor of that period. Occasionally, his dialogue rings false, when he has characters use jargon terms such as "macho image" a full generation before they actually became current. Those who remember the period will feel that he has made a valiant effort but has managed only partial success.

Despite such shortcomings, "The Establishment" can be expected to do fairly well -- if only because the two previous volumes in the series were successful and because many readers will be wondering what happened to some of the people they met in those books. But this concluding novel of the Lavette trilogy does not quite live up to the promise of the volumes that went before it.