n vivo exposure of tumor-bearing mice to the antineoplastic agent 5-(3,3-dimethyl-1-triazenyl)-1H-imidazole-4-carboxamide (DTIC) results in increased immunogenicity of tumor cells, an event often referred to as "chemical xenogenization" (CX). To verify the hypothesis of DTIC-induced somatic mutation(s) as the major mechanism underlying CX, studies were performed to dissociate CX from the onset of drug resistance, which was invoked in the past as an event leading to selection of preexisting immunogenic clones. Therefore, experiments were done with a DTIC-susceptible tumor line treated with DTIC and quinacrine dihydrochloride (Q), an antimutagenic compound, according to selected experimental schedules. At different transplant generations, the CX and the onset of drug resistance were evaluated. The results show that a) Q does not prevent the onset of DTIC resistance, b) DTIC-resistant clones arising after treatment with DTIC plus Q are not immunogenic, and c) CX is selectively antagonized by Q. The present data confirm that DTIC-induced immunogenicity is not the result of a selection mechanism mediated by the drug and give further support to the hypothesis that the molecular mechanism of CX may be related to somatic mutation(s).