A cytostatic composition consisting essentially of a cytostatically effective amount of at least one N-substituted-dioxiranyl isocyanurate having the formula ##STR26## wherein each oxiranyl is the same or different, has not more than 12 carbon atoms and has the formula ##STR27## wherein R.sub.1, R.sub.2 and R.sub.3 are the same or different and members selected from the group consisting of hydrogen, alkyl, haloalkyl, hydroxyalkyl and alkoxyalkyl and n is an integer from 1 to 8, with the proviso that if n is 1 in all oxiranyl groups, at least one of R.sub.1 or R.sub.3 is other than hydrogen, and R has not more than 12 carbon atoms and is a member selected from the group consisting of oxiranyl, as defined above, hydrogen, alkyl, alkenyl, hydrocarbon aryl, hydrocarbon aralkyl, hydrocarbon alkaryl, cycloalkyl and cycloalkenyl, which R may have further substituents selected from the group consisting of hydroxy, halogen, amino, mercapto, alkylmercapto, hydrocarbon arylmercapto, alkylsulfoxy, hydrocarbon arylsulfoxy, alkoxy, hydrocarbon aryloxy, and alkanoyloxy, and a pharmaceutically acceptable carrier therefor.