Mitomycin derivatives having potent antitumor activity having the formula: ##STR1## wherein A is ON--or R.sub.4 N.dbd.N--[wherein R.sub.4 is selected from ##STR2## and optionally substituted heterocyclic groups (wherein R.sub.5 and R.sub.8 are each independently selected from hydrogen, lower alkyl and lower cycloalkyl; R.sub.6 is selected from hydrogen, halogen, hydroxy, lower alkoxy, amino and nitro; R.sub.7 is selected from lower alkyl and lower cycloalkyl; and X is selected from oxygen, sulphur and imino)]; R.sub.1 and R.sub.2 are each independently selected from hydrogen, lower alkyl, lower cycloalkyl, optionally substituted aralkyl, optionally substituted alkanoyl, optionally substituted arylcarbonyl, optionally substituted alkanesulfonyl, arylsulfonyl and aralkylsulfonyl; R.sub.3 is hydrogen or carbamoyl; Y is hydrogen or methyl; Z is selected from hydrogen, methyl and acetyl; and is an .alpha. or .beta. bond.