This study has investigated the in vivo and in vitro cellular immunity of the AKR/J, (AKR/J x C57BL/6)F1 and (AKR/J x DBA/2)F1 mouse strains to a spontaneously arising AKR/J tumor. The study concludes that: 1) there is an excellent correlation between the growth of the tumor in vivo, and the ability of spleen cells from the respective strains to generate both primary and secondary cell-mediated cytotoxic responses in vitro; and 2) one F1 host can be considered a responder against this tumor, whereas the other F1 strains studied appears to be genetically nonresponsive. Possible mechanisms relating to the observed patterns of response are discussed.