Several derivs. of 2,5-diaziridinyl-3-phenyl-1,4-benzoquinone have been synthesized and their cytotoxicities in six different human cancer cell lines (H460, H596, HT29, BE, K562 and A2780) have been detd. It was obsd. that certain phenol-ester derivs. were significantly more cytotoxic in all of the cell lines investigated. These esters were shown to be cleaved by esterases to form a stable meta-phenol and an unstable para-phenol. The meta-phenol was also highly cytotoxic. Several of these compds. were studied in detail using DNA crosslinking, clonogenic, apoptosis and flow cytometry assays. It is proposed that although the phenol-esters and the phenols can efficiently cross-link DNA, this mechanism alone is not sufficient to explain the toxicities of these compds.