2-Oxy-4H-3,1-benzoxazin-4-ones represented by the formula: ##STR1## and the pharmaceutically acceptable acid addition salts thereof, wherein: a is an integer of 0-4;A is a bond, or alkylene having one to eight carbon atoms;R is hydrogen, phenyl, imidazolyl or cycloalkyl having three to six carbon atoms, wherein the phenyl, imidazolyl or cycloalkyl ring is optionally substituted with 1-3 substituents independently selected from the group consisting of lower alkyl having one to four carbon atoms, lower alkoxy having one to four carbon atoms, --N(R.sup.1).sub.2, --NO.sub.2, halo or lower alkylthio having one to four carbon atoms, and,each R' is independently selected from the group consisting of lower alkyl having one to six atoms, lower alkenyl having two to six carbon atoms, lower alkoxy having one to six carbon atoms,lower alkylthio or halo-lower alkyl having one to four carbon atoms, halo, --NO.sub.2, --N(R.sup.1).sub.2, ##STR2## --NR.sup.1 COR.sup.2, and ##STR3## in which each R.sup.1 is independently hydrogen or lower alkyl having one to four carbon atoms, or together form a piperidine or a piperazine ring optionally substituted at the ring nitrogen by lower alkyl having one to four carbon atoms or --CH.sub.2 CH.sub.2 OH,each R.sup.2 is independently lower alkyl having one to four carbon atoms,A is an alkylene group if R is hydrogen,and the pharmaceutically acceptable acid addition salts thereof are useful as serine protease inhibitors in humans and animals.