O6-Methylguanine was measured by a competitive repair assay in blood leukocyte DNA of seven patients with Hodgkin's or non-Hodgkin's lymphoma during therapeutic exposure to procarbazine involving three daily p.o. doses (50 mg each) for 10 days (corresponding to 2.1 mg/kg/day for a 70-kg human). Adduct accumulation was observed in all seven cases, reaching levels up to 0.28 fmol/microgram of DNA (0.45 mumol/mol of guanine). In one individual, maximal levels of adduct were reached after 7 days of exposure, followed by a steady decline, whereas in all other individuals continuous accumulation was observed throughout the exposure period. In four individuals for which data were available for Day 11 (12 to 16 h after the final intake of procarbazine), decreased amounts of O6-methylguanine were observed relative to the last previous measurements. The accumulation of O6-methylguanine was linearly correlated (P less than 0.01) with the cumulative dose of procarbazine, with a slope of 0.011 fmol of O6-methylguanine/microgram of DNA per mg/kg of body weight or 2.68 x 10(-4) fmol of O6 methylguanine DNA per mg/m2. (Two h after the administration of single p.o. doses of 1 to 10 mg/kg of procarbazine to rats, O6-methylguanine formation in leukocyte DNA was just under half that in liver DNA and showed a linear relationship with dose with a slope of 0.017 fmol/microgram of DNA per mg/kg of body weight or 5.67 x 10(-4) fmol of O6-methylguanine/microgram of DNA per mg/m2. A negative correlation (P less than 0.05) between the rate of accumulation of O6-methylguanine in different individuals and lymphocyte O6-alkylguanine-DNA alkyltransferase (AGT) was observed, demonstrating a probable protective effect of AGT against the accumulation of O6-methylguanine during exposure to methylating agents. This observation supports the suggestion of a possible role of procarbazine-induced O6-methylguanine in the pathogenesis of acute nonlymphocytic leukemia appearing after treatment with chemotherapeutic protocols which include procarbazine, based on the finding of low lymphocyte AGT levels in patients with such therapy-related neoplastic disease (Sagher et al., Cancer Res., 48: 3084-3089, 1988). Lymphocyte AGT levels were mainly in the range of 5 to 10 fmol/micrograms of DNA and showed no consistent variation during procarbazine exposure.