Previous studies demonstrated that triazene compounds (TZC) possess antitumor, antimetastatic and immunosuppressive activity, and induce novel antigenic properties in neoplastic cells. Moreover, TZC showed marked antitumor activity in patients with acute myelogenous leukemias (AML). In most cases leukemic blasts with low levels of the repair enzyme O6-alkyl-guanine-DNA alkyltransferase (OGAT) were highly susceptible to TZC. Therefore the cytotoxic effects of TZC against human leukemic cells and the influence of OGAT modulation were investigated. Five leukemia cell lines were treated with the in vitro active derivative of dacarbazine: 5-(3-methyl-1-triazeno) imidazole-4-carboxamide (MTIC), or with temozolomide (TZM), which is readily cleaved to form the linear triazene MTIC in aqueous solution. The results showed that treatment with TZC at concentrations ranging between 62.5 and 250 microM significantly inhibited cell growth of U-937 and K-562 leukemia cell lines, both with undetectable OGAT activity. Growth inhibition was accompanied by DNA fragmentation and reduction of cell volume characteristic of cell undergoing apoptosis. In contrast, Daudi, HL-60 and Jurkat leukemia cell lines, characterized by high levels of the repair enzyme, were resistant to concentrations of TZC up to 500 microM. Treatment of resistant lines with O6-benzylguanine (BG, a specific inhibitor of OGAT) rendered HL-60 and Daudi but not Jurkat cells sensitive to cytotoxic effects and apoptosis mediated by MTIC. The results presented suggest that: (1) apoptosis is involved in cytotoxic activity of TZC; (2) OGAT could have a role in preventing programmed cell death induced by TZC; and (3) treatment with BG could potentiate cytotoxic and apoptotic effects of TZC on leukemic cell lines when high level of OGAT activity is the main factor involved in resistance to TZC.