Many tumours express tumour-specific antigens capable of being presented to CD8+ T cells by major histocompatibility complex (MHC) class I molecules. Current models of antigen presentation predict that the tumour cell itself should present its own MHC class I-restricted antigens to T cells. Earlier cross-priming experiments have demonstrated that at least some MHC class I-restricted antigens may also be presented by bystander cells. There is no detectable presentation of MHC class I-restricted tumour antigens by the tumour itself during priming of tumour-specific responses. The tumour antigens are presented exclusively by host bone marrow-derived cells. These results imply that an efficient mechanism exists in vivo for transfer of MHC I-restricted antigens to bone marrow-derived antigen presenting cells. They also suggest that HLA matching may not be critical in the clinical application of allogeneic tumour vaccines.