We have previously shown that depletion of TCR-V beta 8+ T cells by treatment with mAb reduces the curative effectiveness of low dose melphalan (L-phenylalanine mustard; L-PAM) for mice bearing a large MOPC-315 tumor and extensive metastases. Here we show that V beta 8+/CD8+ T cell lines derived from mice that are in the process of immune-mediated eradication of a large MOPC-315 tumor as a consequence of low dose L-PAM therapy (L-PAM TuB mice) are capable of mediating tumor eradication in vivo upon adoptive transfer. Analysis of the possible mechanisms through which these cell lines bring about tumor eradication revealed that the V beta 8+/CD8+ cells can exert in vitro a potent lytic activity and secrete large amounts of IFN-gamma. Both of these activities can be triggered by the MOPC-315 but not the MOPC-104E plasmacytoma and are restricted by the MHC class I H-2Kd molecule. In vivo neutralization of IFN-gamma by treatment with mAb was found to cause a noticeable delay in tumor rejection in mice subjected to adoptive chemoimmunotherapy with low dose L-PAM and V beta 8+/CD8+ cells; however, all tumors did regress after initial growth. Thus, the V beta 8+/CD8+ cells use an IFN-gamma-dependent mechanism for the realization of their in vivo tumor-eradicating immunity. However, an IFN-gamma-independent mechanism, most likely involving direct V beta 8+/CD8+ CTL activity, is apparently also used.