OUTLINE: This is a dose escalation study of 06-benzylguanine (BG) and carmustine (BCNU). Patients receive BG IV over 1 hour during week 1, and then BG IV over 1 hour followed 1 hour later by BCNU IV over 1 hour during week 3. Courses repeat every 6 weeks in the absence of disease progression or unacceptable toxicity. Cohorts of 1-3 patients receive escalating doses of BG until the biochemical modulation dose l (BMDl) is determined. The BMDl is defined as the dose at which baseline O6-alkylguanine DNA alkyltransferase (AGT) activity in circulating peripheral blood mononuclear cells decreases by greater than 90% at 2 hours after BG infusion. Cohorts of 3 patients receive escalating doses of BG beginning at the BMDl until the biochemical modulation dose t/2 (BMDt/2) is determined. The BMDt/2 is defined as the dose at which AGT activity in human metastatic tumor tissue decreases by greater than 90% at 2 hours after BG infusion. Cohorts of 3 patients receive escalating doses of BG beginning at the BMDt/2 until the biochemical modulation dose t/18 (BMDt/18) is determined. The BMDt/18 is defined as the dose at which AGT activity in human metastatic tumor tissue decreases to undetectable levels at 18 hours after BG infusion. Patients then receive BG IV over 1 hour followed 1 hour later by BCNU IV over 1 hour during week 1. Courses repeat every 6 weeks in the absence of disease progression or unacceptable toxicity. Cohorts of 3-6 patients receive escalating doses of BCNU combined with BG at the BMDt/18 until the maximum tolerated dose (MTD) of BCNU is determined. The MTD of BCNU is defined as the dose preceding that at which 2 or more of 6 patients experience dose limiting toxicity. A cohort of 3 patients receives BG IV over 2 minutes and another cohort of 3 patients receives BG IV over 24 hours. An additional cohort of 6 patients receives BG IV over 24 hours and BCNU IV over 1 hour beginning 2 hours into BG infusion during week 3.