Following an extremely difficult initiation against the top two teams in basketball, the Bullets have played the best defense in the National Basketball Association.

After lopsided losses to Boston and Philadelphia, the group that Gene Shue is molding together has kept five straight opponents under 100 points, yielding an average of 91.2 per game.

Rick Mahorn and Jeff Ruland have anchored the defense, providing the essential shot-blocking ingredient that allows John Lucas, Frank Johnson, Don Collins and others to apply full-court pressure, double-team the man with the ball and trap the receiver of the first long pass.

Mahorn, a muscular 6-foot-10, 235-pounder, has rejected 13 shots in the last four games and averaged 8.6 rebounds despite foul trouble.

Ruland, a 6-11, 240-pound rookie from Iona, has scored 29 points and had 16 rebounds in the last two victories.

"Defense is basically our game," Mahorn said Saturday night after he scored 17 points and blocked four shots to pace the Bullets to a 104-99 triumph over Chicago before 6,715 at Capital Centre.

The victory was the third in the last four games for the Bullets, who may have a difficult time extending their modest two-game winning streak when Milwaukee visits Tuesday night.

"We knew in training camp that it would take a while for our offense to jell, so we all worked extra hard on defense. Right now, that's what's paying off for us."

Ironically, in their only defeat in the last four games, the improving Bullets played probably their best defensive game, holding the fast-breaking Celtics to 90 points. However, after building a 17-point lead in the second period, the offense collapsed when Kevin Grevey pulled a muscle in his left thigh, and Boston prevailed, 90-84.

"Coach has been stressing defense since day one," said Ruland, who has been effective in the fourth quarter of the last two games. The Bullets were clinging to a 77-73 lead going into the final period against Chicago and Ruland enabled them to protect their advantage by scoring nine of his 16 points and leading the team with 10 rebounds.

"We always talk defense when preparing for a game and now that we've had some good defensive games, I think everyone is starting to take more pride in it.

"When Rick and I are in there together, we take up a lot of room around the basket. As long as we're back there, the other guys can press and double-team, which forces mistakes."

With Lucas, Johnson and Collins using their quickness to pressure the ball, the Bullets forced Chicago to make 25 turnovers, which were converted into 30 points.

"If you can't play defense, you can't play for Coach Shue," said Lucas, who has worked hard to improve in that important area. "That's all he talks about."

Shue, naturally, was delighted with his team's performance against Chicago, particularly because he had been concerned about matching up with the Bulls' big front line of 7-2 Artis Gilmore, 6-10 Dwight Jones and 6-9 David Greenwood.

"The Bulls present a special problem because of their size," the coach said. "They are a very good team and are very physical. We just played great defense all night."

Proof that the Bullets aren't giving up many easy shots is the fact that their last four opponents have, combined, made 39 percent of their shots (140 of 352).

Jim Chones made his first significent contribution, scoring eight points in each of the first two quarters to carry the offensive load in the first half . . . Brad Holland, the other part of the Los Angeles connection, provided a big boost in the decisive third quarter with 10 of his 12 points . . . Don Collins led everybody with eight assists . . . The Bulls now are 0-5 on the road . . . Gilmore had 24 points on only 12 shots; Reggie Theus needed 23 to get his 27 points . . . The Bulls got just eight points from their fast break.