Until last week, Jack Nicklaus thought he might have cancer. He won't say the words. He says he has had diarrhea since eating bad crabs in March. He says he went to the Cleveland Clinic last Thursday for sophisticated tests. He'll tell you his father Charlie died of cancer of the pancreas and liver. But if you ask Nicklaus about cancer, he will say a wise-guy thing.

"Bacteria from crabs doesn't generally go into cancer in a couple months," he said today.

"Jack, you're a methodical organized guy," someone said, "and considering what happened with your father, when you make your list of what could be causing the diarrhea, you have to think . . ."

"I went to Cleveland," Nicklaus said, no longer the wise guy but more the frightened child, "because I wanted to get that out of my head."

All Nicklaus wants in his head is the U.S. Open golf Tournament. At 9:36 Thursday morning, he begins pursuit of an unpreceedented fifth Open championship. He won last June with a record 272. Now 41, the father of five teen-age children, winner of 19 major championships, a multimillionaire, Nicklaus yet burns with a competitive fire so intense he wants no spare thoughts this week.

The doctors told him it couldn't be cancer.

They told him it was a bacteria from the crabmeat served at a dinner during the Tournament Players Championship in March.

The doctors said the bacteria might leave in a week, or in a month, or in a year.

So Nicklaus did nothing about it, except take some medicine that didn't change a thing. But when the Open came close, when Nicklaus knew again the urge to make history, he didn't want to be thinking of the way his father died.

Charlie Nicklaus went quickly. The first sign was jaundice. He died in less than a month. This was in 1970, and it was Jack's worst year of golf in a decade. He hadn't won a major in three years; he was fourth on the money list.Some people said he was washed up, a burnt-out case after eight major championships. He was 30.

"When my father got sick, they didn't have the sophisticated tests they do now," Nicklaus said today. "When I went to the Cleveland Clinic, they did all kinds of tests on me. They did a CAT scan of my liver and pancreas. They had me swallowing tubes down into my stomach. They said to just think of the tubes as macaroni and swallow the whole thing."

He went to Cleveland, Nicklaus said, because it was one of only two places in the country sophisticated enough to do the full range of tests he wanted.

The first question at a Nicklaus press conference today dealt with Merion Golf Club's greens.

"Not very fase," Nicklaus said. That will change, he said, and by Sunday the pros will curse them as the roller-coaster marble slabs that make all Opens memorable for testing the courage and delicacy of a champion.

The second question dealt with Nicklaus' trip to the clinic. What's going on, Jack?

"I have to check my watch," Nicklaus said, smiling, "to see home many days left I've got."

No one laughed.

At the March tournament, maybe 50 pros came down sick after the seafood dinner, Nicklaus said. "A lot of them were going both ways, and I was only going one way, and mine didn't stop."

After two weeks, Nicklaus went to a doctor who diagnosed the problem as bacterial. It would go away in time. Nothing to worry about.

As the diarrhea continued, Nicklaus, according to a friend, "was scared to death he had cancer of the colon."

Early last week, he asked for an appointment with Dr. Richard Farmer at the Cleveland Clinic. "My doctor told me they weren't going to find anything, but he would set it up for me. For my own head, I wanted a quick checkup. They did me A to Z. Some very nice tests. I walked out with a clean bill of health. They gave me some pretty strong medicine -- Pepto-Bismol."

Once a kid whistles past the graveyard, he becomes real brave. Today, Nicklaus laughed a lot. Pepto-Bismol, indeed. But friend's say his father's death reminded him of how fragile life is. It was the year of Charlie's death, 1970, that Nicklaus said he would dedicate his work to his dad. He has won 11 majors since. The friends say Jack was counting the days the diarrhea persisted. "He was up to 57 during the tournament at Muirfield," a man said.

The day of the examination at Cleveland, Nicklaus left by his private jet for Merion. "Jack was euphoric when we got here," a friend said.

That afternoon, Nicklaus shot at 65 on the Open course. It served as notice he is physically and mentally ready here. He might have won at Atlanta two weeks ago, finishing ninth with a last-day 72. In 10 tournaments this season, he has been second twice (at the Masters and at Inverrary) and is 18th on the money list with $101,008.

All of which means nothing when it is time for Jack Nicklaus to tee it up for a major championship.

"When I came to the Open last year," he reminded a newspaperman, "You guys had me buried 20 feet deep."

Winless, even unable to make the cut the week before, Nicklaus began the Open with a 63 and won with a record eight-under-par 272. He has finished second in majors 17 times, third nine times.

"Jack, I have to ask this," said the man from the Associated Press. "Who are you picking to win here?"

Nicklaus grinned warmly."Well.

The AP man waited.

"I can't pick anybody but myself, you know that."

One thing more. America came to George Brett's rescue during the last World Series, sending him little tubes of curative, knitting his name into doughnut pillows. And now once again sports fans have risen up to help a hero in distress.

They are sending diarrhea remedies to the world's greatest golfer.

A man today handed Jack Nicklaus a business card with scribbling on it.

"The blackberry brandy recipe," Nicklaus said, sliding the card aside. "Got a hundred of 'em."

He signed.