PHOENIX WHEN the Oakland A's were in Texas late last season, two of the Boyer brothers, Clete and Ken, had a few laughs together. Ken was there scouting the A's for the St. Louis Cardinals, and Clete was there as an A's coach.

''Ken looked great,'' Clete Boyer was saying now. ''But two weeks later he had some chest pains.'' Ken Boyer had lung cancer. But he didn't tell his family, because the father was ill with intestinal cancer. At the father's funeral a few months ago, Ken Boyer's longtime lady friend called Clete aside.

''I thought she was going to tell me they were getting married,'' Clete recalled. ''But she told me what Ken had. I couldn't believe it. Ken was a moderate smoker, but nothing heavy.''

As the Cardinals' third baseman, Ken Boyer was the National League's most valuable player in 1964. He finished his 15-season major league career with the Mets, the White Sox and the Dodgers. Later he was the Cardinals' manager, and he was supposed to manage the Cardinals' farm team at Louisville of the American Association this season, at the age of 50.

''But he just couldn't do it,'' Clete Boyer said. ''He's in Mexico now to take laetrile treatments.'' About a week ago Clete Boyer happened to mention to Billy Martin, the manager of the A's, who are in training here, that Ken's medical expenses already were up around $10,000 and climbing at about $1,500 a week. Ken Boyer's medical bills are not covered under the major league baseball players' pension plan, because he chose in 1974 not to contribute to the health benefit program.

''That's when Billy suggested, 'Let's have a dinner for Ken; let's get all the baseball people behind it,' '' Clete Boyer said. ''Now we've got the date, March 28, and the place, the Brown Derby restaurant in Scottsdale, where our hotel is. And the letter has gone out to all the ball clubs and the commissioner's office. Just talking about it, we've already had some checks from fans.''

The letter is on A's stationery. It reads, in part: ''I have never asked in my lifetime for a favor for myself or anyone else. This will be my first and, sorry to say, my saddest. Whatever is in your heart to contribute on behalf of Ken Boyer, we would appreciate it. If you are unable to attend, you can pledge by phone (602-275-8314 or 602-275-8370). You may also send the donation to:

''The Ken Boyer Fund, c/o Billy Martin, 5999 East Van Buren, Phoenix, Ariz. 85008. ''All of our thanks in the field of sports, entertainment, life and humanity. (signed) Billy Martin, Chairman.'' Clete Boyer phones his brother every day from the A's training camp. ''The doctors in Tijuana don't want him to get too emotional, so I'm the only one in the family calling him,'' Clete said. ''Everybody else goes through me. But last week when I told him about the dinner on the phone, he broke down, and then Billy and I sat around and cried.

''I went down to see him a few weeks ago. He's eating organic food and drinking a lot of juices - lettuce juice, spinach juice, carrot juice mixed with liver juice. Maybe it'll work. There's a lady there who was given up for dead three years ago, but she's in good shape now.''

Of the 13 Boyer children who grew up in Alba, Mo., another brother, Cloyd, was the first to make the big leagues. He pitched mostly for the Cardinals and is now the Kansas City Royals' pitching coach.

''But my idol always was Ken,'' Clete was saying now. ''He was closer to me in age, about six years older. I was the batboy for the Alba Aces in the Ban Johnson League when Ken was about 16, the best player in the league, even better than Mickey Mantle, who was with the Baxter Springs Whiz Kids over in Oklahoma, about 30 miles away. Mickey didn't blossom until later.''

Ken and Clete played against each other as the third basemen in the 1964 World Series between the Cardinals and the Yankees. ''Ken's the one who beat us,'' Clete Boyer said with a smile. ''We were up, two games to one, and we were leading, 3-0, in the fifth inning when Ken hit a grand slam off Al Downing at Yankee Stadium to win that game, 4-3, and turn the Series around. When he hit that homer, I loved it. In my heart, I think I was pulling for him that year because it was his first Series.''

The brothers had a few laughs during that World Series, too. ''In one of the St. Louis games,'' Clete Boyer said, laughing, ''a grounder went through my legs, and afterward Ken sent me a note saying, 'Watch this infield, you get a lot of bad hops.' And when he dropped a pop fly there later on, I sent him a note saying, 'Watch this air, you get a lot of bad hops.' We always had a lot of fun even though we were fighting to win ball games. But now he's fighting for something else, and I hope he wins this one, too.''

According to his brother, Ken Boyer's highest salary was about $65,000, in 1965. ''If he was playing now, he'd be worth $1 million easy,'' Clete Boyer said, ''but he never made that kind of money. Nobody did then. And he's got expenses. He went through a divorce. And of his four kids, the oldest one, Susie, is married, but David, Danny and Janie are all at the University of Missouri now.''

Clete Boyer's regret is that he never had an opportunity to play with his brother on the same big league team. ''Where we grew up, down in the southwest corner of Missouri, the Cardinals were like a religion,'' he said. ''They signed Cloyd and Ken, but when I came along - I was a shortstop then - they had a bonus kid a shortstop, Dick Schofield, who they had to keep on the big-league roster. So they weren't that interested in me. I signed with Kansas City and then got traded to the Yankees later.

''As a kid I had always fantasized about us being on the Cardinals together, him at third base and me at shortstop. Later on, when I was playing third base, I was still hoping to be on the same team with him. He could play the outfield, too. That would've been something, the two of us on the same team. But it never worked out. We just never were on the same team together.''

Not until now.

Kenton L Boyer