It has been demonstrated that significant protection against YC8 lymphoma can be induced in mice preimmunized with normal DBA/2 spleen cells. The DBA/2 spleen cells used as alloantigens share minor histocompatibility determinants with the YC8 tumor. We have used this model to investigate the nature of the immunity conferred by treatment with the alloantigen and infer that the conditioned resistance observed was maintained by the same effector mechanism. The results demonstrated that repeated immunization of tumor bearing mice with the alloantigen had some beneficial effect as shown by the slower rate of growth of the tumor, and an increase in median survival time over controls. The observations showed however that once tumor was present in vivo, the use of potent tumor specific vaccine can help in increasing survival but can no longer produce high incidence of regressions and cures. Conditioning can potentiate the effects of this treatment by increasing survival and cure.