Accessory cell activity of spleen cells from M-MuLV neonatally injected (tolerant) mice was studied to evaluate their ability to take part in in vitro CTL generation against tumor-associated antigens induced by M-MSV. In contrast with accessory cell activity in normal spleen cells, spleen cells from M-MuLV-tolerant mice are unable to reconstitute the in vitro virus-specific CTL generation of M-MSV immune, Ia-depleted spleen cells. A selective defect seems to characterize M-MuLV-tolerant mice as their spleens constitute a good source of accessory cells for alloantigen CTL generation.