A triazene-xenogenized tumor sub-line was derived from the mouse mastocytoma cell line P815 following several transplant generations in vivo on DTIC. The highly immunogenic P815/DTIC variant line expressed new CTL-defined antigens. Novel antigens were also detected by antibodies in immunoprecipitation and by Western blot analysis. Upon immunoprecipitation of metabolically labeled cells, one such variant-specific 20-kDa antigen was shown to be related to retroviral envelope protein p15E. When injected intrasplenically into recipient mice, the electroblotted nitrocellulose-bound 20-kDa antigen resulted in increased frequency in CTL precursors to P815/DTIC cells. In addition to previous data in the L5178Y/DTIC tumor-model system, these data suggest that expression of aberrant, retrovirus-related proteins may be a common finding in different parental tumors xenogenized by triazene treatment.