James Roosevelt, who was trained by FDR, the Marine Corps, the House of Representatives and three wives, is a man of 72 in rosy health who fears neither man nor beast nor cholesterol and who, moreover, can conduct intelligent conversations before 8 in the morning.

"This book of yours," he was reminded.

"Well, yes," he said, raising his eyes from a ton of scrambled eggs and bacon in the dining room of the Jefferson Hotel where everyone else was whole-wheating. "I thought writing a novel would be fun and it was. It's fiction, but one thing I wanted to show was the strains that affect a family in which both parents are in public life."

All the names are real -- Stalin, Churchill, all the Roosevelts; and the action is plausible though fictional.

"What about Dr. Einstein?" he was asked.

In "A Family Matter" James Roosevelt is a character who represents his father, the president, in matters of such sensitivity as the atom bomb secret. In actual fact, he had no role in the Manhattan Project, but the book has a scene or two of utter credibility in which young Roosevelt and Einstein discuss the forthcoming bomb.

"You know," he was warned, "someday people are going to rely on your novel for their facts."

The only thing anybody knows about Richard II or Richard III or King Lear is what a writer (Shakespeare) made up, and it does little good to say he got this Richard or that one all wrong.

The new Shakespeare looked up from his bacon modestly and said:

"That's why I'm going all around telling everybody I can that I have written a book of fiction."

Not that young Roosevelt (as it's hard to keep from calling him) is himself a writer. He's more a man of action, honored highly for bravery in Marine raids, a retired Marine Corps general.

He had unique knowledge of the FDR White House and the president's mind and way of operating. Even in fiction, he felt safe enough in judging what FDR's position would be, and of course the entire Manhattan Project is well documented, including the views of Einstein and other sceintists. So he found a writer, Sam Toperoff, who pounded out the actual words from material dreamed up (with plenty of convincing detail) by Roosevelt.

Unlike most sons of major politicians, young Roosevelt was taken in hand by his father and given important responsibilities as the president's secretary while still in his 20s. In his early 30s he held a lieutenant colonel's commission in the Marines arranged to give him appropriate status as President Roosevelt's assistant on a South American tour, a rank he resigned when he saw World War II coming, on the grounds he was too young and inexperienced for such a rank. Instead, he entered combat duty at a lesser grade and acquitted himself valiantly.

President Roosevelt did in fact use him as a personal envoy to heads of state in the Middle East, at a time when Hitler was sweeping all before him. His mission was to relay the president's exhortation not to commit their countries to the German cause, since the tide would turn, the Allies would win the war, and they'd be extremely sorry if they had supported Germany. This was done, young Roosevelt observed, while the Neutrality Act was in force. But sometimes, of course, you have to use common sense.

Roosevelt lost most of his hair as quite a young man, though what remained has faithfully remained, and in more recent years he has lost his dimple too, but no way could be devised to ask him if he missed it. He was a good-looking youngster and is a good-looking oldster.

Ronald Reagan, the Republican presidential candidate, has taken to quoting Franklin D. Roosevelt, and young Roosevelt was asked if this offended him, as it offended some of the more pious Democrats.

"Why should it?" he asked. "Even a Republican can learn.

"Besides, though I have come to no conclusions on the candidates, I remember when Reagan was a liberal, and I think he's still far more liberal than he's made out to be. I don't think he's some kind of right-wing madman bound to get us into war. Though I suppose that's the strategy that will be used against him in the campaign. I think Reagan is a sincere man, and sincere in quoting my father. He didn't say, you will notice, that he endorsed the whole Roosevelt agenda."

James Roosevelt's first two marriages ended unhappily. His wife, Mary, is a red-haired former teacher of first-grade pupils. She was teaching at an international school associated with the United Nations, and was English. aNow she is becoming an American, and as her husband notes, will now have the pleasure of canceling out his vote at the polls. As wives do.

He lives out from Los Angeles, a part of California he represented for six terms in the House, resigning to accept a U.N. post urged on him by President Lyndon Johnson. Later President Ford appointed him to the Commission on Executive Exchange, a post he still holds. The "executives" of the title are both government and private-sector executives moving from one of their respective sectors to the other for a year's experience. It can be wonderful, Roosevelt said, how understanding enlarges when you find yourself on the other side of the fence. There's a scene in the novel in which James reproaches his mother, Eleanor, with not having been a conventionally good mother.

"It must have been hard to hear that," an interviewer suggested, and he said:

"Yes. In her autobiography she devotes a chapter to it. She was sorry to have failed in some ways as a mother, but she saw no way out if she was to live her life for the causes she believed in. It's the way it was, and she was sorry, but she did it as well as she knew how."

"But you feel no bitterness now, do you?" he was asked.

"Of course not. I've had an interesting life, partly because of my parents. Sometimes people expect sons of famous parents to at least equal them, on the grounds they've had every conceivable advantage, and therefore ought to be at least as successful. I remember one lady who voted for me in California said she expected me to do at least half as well as FDR. So you never escape comparison with your parents."

"The older you get the more sympathy you usually develop for your father," it was suggested, "and in your case life has shown you some of the pressures on a president and his wife."

"Yes. Even in such things as the total lack of privacy. I remember for instance the shock we felt when that book came out about Mother and the woman who was supposed to be her lover. One member of the family phoned me and said we should bring a lawsuit immediately. I said, 'What, and sell another 50,000 books through publicity of the lawsuit?"

"In England they have a council of some kind, and when a publisher or paper is not sure whether the facts justify publication, the council will pass on it. It's tempting to wish we had such a council here. But I guess in the long run we're safest the way things are."

"Sooner or later," his visitor said, "you have to trust plain people to do their own sorting out of what other people say, and maybe our system of letting any ass publish what he wishes is as good as any other. Eleanor Roosevelt's reputation doesn't seem to have suffered, no matter what was printed."

"Only this," he said "that after that book [suggesting lesbianism] I got a lot of letters from women who said Eleanor had been their idol, and how cruel it was to have their idol smashed. People who are thinking of public life are bound to notice this sort of thing and maybe think twice."

But nothing stops the Roosevelts.

"Didn't we read you have been a Kennedy delegate this year?" he was asked.

"Not me," he said. "My son, Jim Jr."