Mouse tumor cells transfected with syngeneic MHC class II genes are highly immunogenic in the autologous host, and induce a potent tumor-specific immunity against wild type tumor. Previous studies with sarcoma tumor cells expressing transfected class II gene products with truncated cytoplasmic domains suggested that during the process of tumor rejection costimulatory molecules are induced on the tumor cells, contributing to the cells' ability to stimulate immunity. In the present study we directly demonstrate that tumor cells containing full-length class II heterodimers are induced to express B7-1 and B7-2 costimulatory molecules during the rejection process. In contrast, tumor cells expressing class II heterodimers truncated for their cytoplasmic tails are not induced to express B7-1 and/or B7-2. Blocking the interaction of the induced costimulatory molecules with their corresponding receptors on T cells prevents tumor rejection. These results support the hypothesis that the cytoplasmic domain of the MHC class II molecule is involved in induction of costimulatory molecule expression, perhaps via intracellular signalling pathways. Because class II, B7 transfected tumor cells are such effective immunogens against ascites and solid tumors, they have also been tested in metastatic disease. K1735 and B16BL6 mouse melanomas, when transfected with syngeneic MHC class II and B7-1 genes, are significantly less metastatic than parental cells, and immunization with the transfectants protects against subsequent challenge with wild type tumor.