LOU DIMURO had a reputation for bad luck. His fellow umpire Larry Barnett recalls with a catch in his voice that ''I used to tell Lou, 'If you didn't have bad luck, you wouldn't have any luck at all.' ''

DiMuro had been knocked down at home plate by Cliff Johnson, one of the biggest men in the American League, and had missed nearly a season with injuries. He had fallen in a damp runway soon after his return. Last year he missed time with a hernia, and early this season he had to go home because of high blood pressure.

''The thing about Lou was that he always apologized,'' says Barnett, who was DiMuro's crew chief. ''When his blood pressure was better, he said, 'I'm sorry you had to work harder.' ''

Barnett was recalling his friend Monday night in a hotel room in Milwaukee. There was a game that night, but other umpires were filling in. A priest friend was consoling Barnett and Mike Reilly and Durwood Merrill on the loss of their friend, Lou DiMuro, who died early Monday morning after being hit by a car in Arlington, Tex.

''I can hear Lou now,'' Barnett said. ''He's saying, 'Geez, I'm sorry to put you guys through this.' '' DiMuro was a major league umpire for 19 years, and, while he did not exactly die in the line of duty, he certainly did die far from his wife and five children, walking the city streets alone, exercising the arthritic hip that was a souvenir of his accident at home plate.

An accident had placed DiMuro in his lonely profession, he recounted nearly a year ago. Umpires were under siege last June, with Billy Martin throwing dirt at one man and Jimmy Piersall, the announcer for the Chicago White Sox, inciting fans against another umpire. It seemed like a good idea for a journalist to learn a little bit more about the umpires behind the barricades of a tiny room under Yankee Stadium.

DiMuro, who was going to be the home-plate umpire that night, began work by rubbing New Jersey river mud onto the glossy new baseballs. A tall man with dark hair, he was then 50 years old and the oldest man in his crew.

''I grew up in Bay Ridge, went to high school in New Jersey, played ball in Germany for three years in the Air Force,'' he said. ''I never took notice of what umpires did until I broke my finger playing ball.

''They asked me to umpire a game with a cast all the way up to my elbow, so I did. The first thing I noticed was that my buddies would holler at me during the game, and even that night back in the barracks.

''But I got to like umpiring, so when I got out of the Air Force I went to the Bill McGowan School and went to the minor leagues for eight years. Many times I've been escorted out of town by state troopers - Mayfield, Ky., Union City, Tex., Fulton, Ky., cars following you. But you improved, and the players improved.''

DiMuro reached the American League in 1963 and gained the reputation as a calm umpire who did not feel the need to dominate managers.

''Some guys take it more seriously than others,'' he said. ''Once you develop a persecution complex, you're in trouble. You've got to let the managers have their say. You don't want to cut off all arguing.

''This is the great national pastime. Some great general, maybe Patton, once said, 'It's great to live in a country where you can boo the umpire.' I agree with that.''

The only rancor the umpires showed was about minor league umpires who had worked during a strike in 1979. They felt bitter that socalled ''scabs'' had been given permanent jobs while a respected umpire like John (Rocky) Roe had been kept in the minors. In a strange twist, Rocky Roe will be DiMuro's replacement in the Barnett crew.

Until the collision with Johnson, Lou DiMuro's most dramatic moment had come in the 1969 World Series, when Cleon Jones of the Mets argued that he had been hit in the foot by a pitch.

The ball went into the Met dugout, and Gil Hodges, the Met manager, trudged out mesmerizingly slowly. When Hodges got to home plate, he unfolded his huge paw and produced the ball - a ball - with an apparent smudge of shoe polish on it. He said slowly, ''Lou, the ball hit him.'' DiMuro stared at the ball and waved Jones to first base, and Donn Clendenon hit a home run to put the Mets back in the game. Twelve years later, DiMuro said: ''I had a play just like it during the 1969 season. I watched the ball go into the Met dugout. Nobody did anything to it. I made the right call.''

He was confident about his own work but concerned about the conditions of umpiring. Watching a televised replay of Billy Martin throwing dirt on Terry Cooney made DiMuro ''feel physically sick.''

''This could start a precedent,'' he said. ''We've seen bottles come out of the stands. What will the next step be?'' DiMuro had seen his own life altered by the Johnson collision. He spent most of the 1979 season at home with his wife, Sue, and their four sons, now ranging from 10 to 17 years. In 1980, he and Sue had a fifth child, a daughter.

''You see what happens when you stay home for a year?'' joked Nick Bremigan, a member of DiMuro's crew last year. DiMuro spoke proudly of his wife for rearing the children during his long absences. He said: ''You have to have a certain kind of wife. She knew what it was like, because I was already in the minors when we got married. It's difficult. You call home. You try to discipline the kids. But it's hard.''

Last Thursday, Mike Reilly was supposed to play golf with DiMuro on a day off between Anaheim, Calif., and Arlington, Tex., but DiMuro found a way to get home to Tucson for a day. On Sunday night, DiMuro went for a walk and died. Larry Barnett had to break the news to Sue DiMuro, and Durwood Merrill had to identify the body at the hospital. The next morning, Al Clark and Jim Evans, two other umpires who had just come to town, went out to the ball park to pack DiMuro's bag.

''You know what they found?'' Barnett said. ''A picture of his father on top of his address book, letters from his kids and presents for all his kids. That was just like Lou. Lou was only gone from home three days, but he had already bought presents for his kids.''

Louis Dimuro (1931)