The activity of the enzyme O6-alkylguanine-DNA-alkytransferase (AGAT) protects cells from the cytotoxic effects of alkylating agents. This Phase II trial was designed to assess the efficacy of a strategy designed to modulate the resistance to carmustine (BCNU) mediated by AGAT using streptozocin (STZ) in patients with advanced refractory melanoma. Seventeen patients who had failed prior chemotherapy were treated with STZ at 500 mg/m2 daily for 4 days with BCNU at 150 mg/m2 on day 3. Peripheral blood lymphocytes for assay of AGAT activity levels were collected prior to therapy and following the third dose of STZ. There were two partial responses in the 15 patients evaluable for response (13%). Most patients received only a single cycle of therapy due to rapidly progressive disease. Two patients developed fatal pulmonary toxicity, and one developed myelodysplasia. Other toxicities included transient rises in liver function tests. AGAT levels decreased by a mean of 53% in 9 patients but actually increased over baseline in 3 patients while on therapy. Based on these data, BCNU and STZ are not an effective combination for the therapy of advanced refractory melanoma, and pulmonary toxicity due to this combination appears to be increased compared with BCNU alone. STZ is not an effective modulator of AGAT activity when given on this schedule. New strategies designed to deplete AGAT activity using O6-benzylguanine or temozolomide should be explored with careful attention to the possibility that this approach may potentiate both the toxicity and efficacy of BCNU.