The Washington Bullets, with their season at stake, went into Boston Garden tonight and defeated the mighty Celtics.

They withstood every Boston surge by refusing to be intimidated, even under all those championship banners and with a sellout crowd of 15,320 screaming at them.

All five starters scored in double figures as the Bullets controlled the tempo throughout, then held their poise at the end and came away with a 107-99 victory that sets up a showdown with the Atlanta Hawks Saturday at Capital Centre at 8:30 p.m.

The Bullets failed to gain ground on Atlanta or New York, however. The Hawks routed Chicago, 138-101, to stay 1 1/2 games ahead of the Bullets in the Eastern Conference and New York beat San Antonio, 102-100, in overtime, to stay a game in front. Only two of the three can make the playoffs.

"We kept our poise, even when they were making those runs at us," said Bullets guard Frank Johnson. "That, and the fact that we limited them to one shot, were the keys. We played the game at our tempo. When the break was there, we took it, and when it wasn't, we took it down low. When they stopped that by double teaming, we punched it out and hit the jump shots."

The Bullets outrebounded Boston, 45-40, and limited the Celtics to four offensive rebounds, getting 13 themselves.

The Celtics, who had won five in a row, scored six straight points, four by Robert Parish, to take a 63-58 lead with 4:42 left in the third period.

The Bullets turned the game around by outscoring Boston, 16-6, the remainder of the quarter to take a 74-69 lead into the final 12 minutes.

Guards Johnson and Ricky Sobers were at their best in that stretch, as Sobers had four points and two assists and Johnson had four points and one assist. Jeff Ruland also had four points as the Bullets held Boston to one field goal in that span.

The Celtics made two runs at the Bullets in the final quarter. Scott Wedman and Gerald Henderson led the first rally that closed them to 80-78 with 8:47 left, but Joe Kopicki rallied the Bullets in the next two minutes. He got successive offensive rebounds, put in a basket over Parish and fed Greg Ballard for a 16-foot jump shot.

Sobers then stole the ball from Henderson and scored on a breakaway to increase the lead to 86-78.

The two sides traded baskets most of the rest of the game. After a free throw from Kevin McHale and a 20-footer by Larry Bird, the Bullets' lead was down to 99-95 with 1:08 left. Ruland made one free throw and Wedman made a jumper to cut it to 100-97 with 44 seconds left.

The Bullets got a fast break to Sobers off the inbounds play, however, then shut off the inside and forced the Celtics to attempt long jumpers.

They missed and the Bullets got the rebounds and were usually fouled as well. Washington made five of six free throws in the final 33 seconds.

This was a night for the Bullets' guards. Sobers had 27 points and eight assists and made all eight free throws. Johnson had 15 points and nine assists and made his six free throws.

"We knew that if we played together and executed, we could beat them," said Sobers. "This is a very satisfying win and a big confidence builder. If we can beat Boston up here we know we can beat anybody, anywhere."

The teams split six games this season.

Ruland, despite reinjuring his right wrist, played 41 minutes and had 22 points and 10 rebounds. He and Rick Mahorn took turns posting up inside and both showed an uncanny ability to know when to shoot and when to pass the ball back outside.

"We played excellent defense and we got the high-percentage shots," said Washington Coach Gene Shue. "That was our game plan. The Celtics are a great team, and we knew we couldn't compete with them if we just ran up and down the floor. We had to grind it out."