2,5-Piperazinedione, 3,6-bis(5-chloro-2-piperidyl)-,dihydrochloride (NSC-135758) inhibited DNA synthesis but not RNA and protein synthesis in Adenocarcinoma 755 cells in culture. The expression of such inhibition was delayed in time; it was necessary to expose tumor cells to NSC-135758 for 10-12 hours before measuring the macromolecular synthesis in order to demonstrate selective inhibition of DNA synthesis. Inhibition of DNA synthesis was demonstrated to be irreversible in human epidermoid carcinoma cells in culture. Exposure of cells in suspension culture to NSC-135758 or to melphalan for 1-4 hours, and then incubation of cells in the absence of an inhibitor for 20 hours, resulted in preferential inhibition of DNA synthesis; inhibition of RNA synthesis was observed under these conditions but was less pronounced. Chemical evidence for alkylating activity of NSC-135758 and of the bis-aziridine derived from it (NSC-201424) was obtained by demonstrating their reaction with 4-(p-nitrobenzyl)pyridine. NSC-135758 was more potent as a cytotoxic agent than was its derivative, a result which suggests that NSC-135758 is the active alkylating agent. Reaction of NSC-135758 with diethylamine was examined; the product obtained upon alkylation of diethylamine by NSC-135758 was identified from its proton magnetic resonance spectrum and by field desorption mass spectral analysis. These results support the view that NSC-135758 acts as an alkylating agent in inhibiting DNA synthesis and cell proliferation of tumor cells in culture.