The treatment in vivo with anti-tumour drugs can induce an antigenic alteration of L1210 leukemia resulting in the rejection of an inoculum of 10 X 10(6) viable cells in syngeneic mice. As drug-induced antigens appeared in excess of any pre-existing tumour-associated transplantation antigens (TATA), viable altered cells have been used to sensitize syngeneic animals. Experiments showed that viable altered cells elicited stronger anti-TATA reaction than X-inactivated parental cells, as measured by host survival to a challenge of L1210 leukemia. TATA immunogenicity of parental cells has been preminent, in the strain of animals used, to determine the sensitizing effectiveness of 5-(3,3-dimethyl-1-triazeno)-imidazole-4-carboxamide (DTIC) cells. Host protection to an inoculum of parental tumours has been more proficient with the DTIC subline derived from highly immunogenic L1210Ha cells than from poorly immunogenic L1210Cr cells. Immunoprophylactic inocula, proper chemotherapeutic treatments and adoptive transfer of immune lymphocytes used in combination exerted a synergic host protection.