Doug McMahon, 18, and his brother John, 16, shot a five-under-par 67 yesterday at Alexandria's Belle Haven Country Club to lead the field after two days of qualifying rounds in the Belle Haven Four-Ball amateur golf tournament.

The McMahons, students at Fairfax County's Fort Hunt High School, never carded a bogey in the best-ball event. They birdied the first, seventh, 14th and 15th holes.

John McMahon was 1981 winner of the Mid-Atlantic PGA Junior Championship and Doug won the Tar Heel Junior Championship.

Butch Mielke of Wicomico Country Club and Buddy Peoples of Crofton Country Club shot 68 for the second-best score in qualifying play. They shot a three-under-par 33 on the front nine, then picked up another stroke on par on the back nine.

Tied for third with scores of 69 were the teams of Gill FitzHugh of Army Navy and Marty West of Columbia and Jamie Gough of Army Navy and Bob Hullender of Andrews Air Force Base.

Eight teams yesterday scored better than the five teams that tied for best score in qualifying play Thursday with a one-under-par 71.

Six more teams shot 71 yesterday, necessitating an 11-team playoff for the seven remaining positions in the 16-team championship flight match play beginning this morning. Last year's winners, Randy Morris of Washingtonian Country Club and Brian Willard of Congressional, automatically qualify for the championship flight.

The tournament concludes Sunday afternoon.

Alwin Celebrates With 67 -

Mark Alwin of Congressional celebrated his 32nd birthday yesterday with a five-under-par 67 to win the Chantilly Pro Am.

Alwin, whose five birdies earned him the low pro score of the season, beat out Mel Rifman of BWI Range and Bill Winters of Robindale, each of whom had 70.