Bis(trans-2,3-dimethylaziridinyl)phosphinyl urethane (I) was synthesized and compared with the corresponding cis-2,3-dimethyl derivative (II).The comparative alkylating activities and rates of hydrolysis of these two stereoisomeric aziridine derivatives, I and II, were determined and compared with corresponding data for the monomethyl derivative (III) and two other clinically tested members of this series of antineoplastic agents (dual antagonists), AB-100 (IV) and AB-132 (V).The structures of the final hydrolysis products of I, II, and III were determined and confirmed by direct synthesis.The results indicate that the mechanisms of hydrolysis of I, II, and III (as that of the unsubstituted aziridine derivative IV) are essentially SN2, in contrast to the much faster hydrolysis of the 2,2-dimethyl-aziridine analog (V) which involves a carbonium-ion mechanism.These studies give further support to the hypothesis that the unique pharmacol. properties of V, as compared to other members of this series, may be related to the unique chem. properties of the 2,2-dimethylaziridine moieties.