4H-3, 1-Benzoxazin-4-ones are alternate substrate inhibitors of the serine proteinase human leukocyte elastase (HL elastase) and form acyl enzyme intermediates during enzyme catalysis. We have synthesized a large variety of benzoxazinones using specific methods that have been adapted to achieve the pattern of ring substitution dictated by theoretical considerations. The results of the inhibition of HL elastase by 175 benzoxazinones are ...