The devastated Philadelphia 76ers had better study the films of the recent Washington-Boston series before they go against the Celtics in the second game of the NBA Eastern Conference final.

The 76ers tried to run with the world champions today and got chewed up in a buzz saw as the Celtics built a 14-point lead midway through the second quarter and raced to a 121-81 victory. A sellout crowd of 15,320 howled in delight.

With Larry Bird getting 15 and Robert Parish 14, the Celtics dominated the rebounding, 67-50, while running up their greatest margin of victory ever in their long and illustrious playoff history.

The Bullets provided a much better match against Boston in the semifinals because they were willing to forego the fast break, slow the pace and force the Celtics into a half-court game.

"It was a great feeling to get running again," said Bird, who shared scoring honors with Parish with 24 points. "No more of that walking up and down the court like we did against Washington."

Philadelphia never has been very effective in a pattern-oriented game, but it appears that will be its only chance of slowing the onrushing Celtics when the series resumes here Wednesday night. The following two games will be Saturday and Sunday afternoon in Philadelphia.

The big question now is will Darryl Dawkins return and how effective will the 76ers' big center be? He sat out the second half, saying his leg was sore, after getting just two rebounds before intermission.

"He said his leg was hurting," Coach Billy Cunningham said when asked why Dawkins didn't play in the second half, particularly after Caldwell Jones picked up his fifth foul with 9:56 left in the third quarter. "What can you say to someone who says he's hurting?"

Dawkins suffered a double fracture of his right fibula bone Jan. 17 and missed 28 games before returning March 24. He has played inconsistently since and says his leg still is bothering him.

"I'm going to have X-rays taken tomorrow," he said after the game. "The pain is getting worse. I don't know when I aggravated it, but when I look up and see (Rick) Robey scoring on me and beating me up and down the floor, I know I'm not helping the team. I just hope there is something I can do. I don't know how it will be Wednesday."

With Dawkins on the bench and Caldwell Jones fouling out after getting just three rebounds in 12 minutes, the 76ers had no chance against the Celtics' bulk in the second half. It was a mismatch of mid-season proportions.

"I certainly didn't anticipate a game like this," Boston Coach Bill Fitch said. "It was just a great game for us. Everything went our way. The timing on our plays was perfect."

Cunningham, obviously frustrated after sitting through a humiliating second half when the Celtics' lead ranged from 30 to 48 points (110-62 with 6:41 left), angrily called a timeout after rookie Charles Bradley's slam dunk with 1:09 to play and shouted "showboat" at the Celtics' bench. After cooling down, he refused to comment on the incident.

"We weren't trying to rub it in," Fitch said. "You don't rub it in in this league when you have six more games to play. But it's tough to hold back because you have to put it up every 24 seconds."

The unlikely hero for the Celtics was Robey, who scored only 26 points in six regular-season games against Philadelphia. He relieved Parish with two minutes elapsed in the second quarter and scored 15 points to help the Celtics increase a six-point lead to a insurmountable 62-45 advantage at halftime.

"We kept going inside to me and I took advantage of it," Robey said. "I got some extra playing time at the end of the regular season and now I think my teammates have more confidence to get the ball to me."

A well-rested Parish scored six quick points early in the third quarter and when Caldwell Jones fouled out, Philadelphia had no one to defend against the Celtics' huge front line, which combined for 91 points.

"We have to do a better job rebounding," Cunningham said. "There's no secret to that. They hurt us so many times on second shots. We just can't let ourselves get in foul trouble.

"They hurt us in every phase of the game. You can go from A to Z. But it's just one game. I think we can come back. We've been here before. We know what it takes to win."

Andrew Toney, a nemesis to the Celtics in last year's playoff, kept Philadelphia in the game early with nine points in the first quarter. However he finished with only 15, making five of 12 shots. Julius Erving had 12 points at halftime, but played only eight scoreless minutes after intermission before sitting out the rest of the travesty.

The Celtics blocked 11 shots, seven by Parish . . . Five of Robey's six rebounds were offensive . . . The 76ers, forcing their shots throughout the second half, made only 30 of 93 attempts (32 percent) . . . Bird led everyone with 10 assists and four steals . . . Philadelphia made only 62 percent of its free throws . . . "The 76ers were just a step slow all day," said Nate Archibald.