1-(phenylmethyl)-4,7,10-tris-[(4'methylphenyl)-sulfonyl]-1,4,7, 10-tetraazacyclododecane proved to possess potent in vivo antineoplastic and in vitro cytotoxicity in murine and human tissue cultured cells. In Tmolt3 leukemic cells 1-(phenylmethyl)-4,7,10-tris-[(4'methylphenyl)sulfonyl]-1,4,7, 10-tetraazacyclododecane inhibited the activities of the regulatory enzymes of the purine pathway leading to significant reductions in both RNA and DNA synthesis. Other sites of action of the compound which led to reductions of nucleic acid synthesis were suppression of nucleoside kinase and thymidylate synthetase activities and reductions in the d[NTP] pool levels. DNA fragmentation occurred which should be linked directly with cancer cell death.