A number of trials were conducted to determine the effect of O6-benzylguanine pretreatment on the sensitivity of human colon tumor xenografts to the antitumor effects of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). O6-Benzylguanine has been shown to inactivate the DNA repair protein, O6-alkylguanine-DNA alkyltransferase (AGT), which is primarily responsible for resistance to alkylnitrosoureas including BCNU. Colon tumor xenografts carried in nude mice were analyzed for their AGT content, and tumors with low, intermediate and high levels were chosen for further study. The AGT activity of HC-1, GC-3, VRC-5 and CX-1 human colon tumor xenografts was 16, 113, 180 and 367 fmol/mg protein, respectively. Treatment of mice consisted of vehicle alone, 6.25 to 50 mg/kg BCNU administered alone or BCNU (6.25 to 25 mg/kg) 1 hr after 120 mg/kg O6-benzylguanine on days 7 and 14 post-inoculation. Toxicity studies revealed that pretreatment with O6-benzylguanine increased the toxicity of BCNU, requiring administration of about 4-fold less drug. The growth of the VRC-5 tumor at day 42 post-inoculation was inhibited by 39% following treatment with 12.5 mg/kg BCNU alone and 92% when BCNU was combined with O6-benzylguanine pretreatment. The combination of O6-benzylguanine and BCNU (12.5 mg/kg) at day 42 resulted in an inhibition of HC-1 and CX-1 tumor growth by 84 and 72%, whereas BCNU alone inhibited growth by 54 and 14%, respectively. Therefore, the degree to which the antitumor effect of BCNU was increased by O6-benzylguanine pretreatment was dependent on the AGT activity, with a greater effect in tumors of intermediate or high activity. These data suggest that there is a role for O6-benzylguanine combined with BCNU in the treatment of human colon tumors.