<p>Mitomycin derivatives having potent antitumour activity having the formula: <CHEM> wherein X is <MATH> (wherein Ra, Rb and Rc each independently represent lower alkyl, cycloalkyl having 3-7 carbon atoms or phenyl; or Ra and Rb are bonded together to form a poly-methylene group having 2-5 carbon atoms); one of R1 and R2 represents carbamoyloxymethyl and the other represents hydrogen; or R1 and R2 may together form an exocyclic methylene group; and Y and Z each independently represents hydrogen or methyl.</p>
