While immunization with allogeneic spleen cells did not generate positive cytotoxic activity, it produced accelerated rejection of subsequent tumour grafts carrying the same H-2 antigen. No augmented generation of cytotoxicity was detectable by 51Cr-release assay in the host spleen cells, even in the presence of accelerated rejection of tumour allografts. However, augmented cytotoxicity was generated in mixed lymphocyte culture and in peritoneal lymphocytes after an intraperitoneal boost. These results indicate that while immunization with allogeneic spleen cells does not generate mature cytotoxic T lymphocytes (CTL) detectable by the present assay, it may produce premature CTL that rapidly differentiate into mature CTL after direct contact with antigen at the site of graft rejection. The inability to generate a high degree of cytotoxicity in the spleen cells may be ascribed to the early development of CTL at the rejection site. The relationship between accelerated rejection of allogeneic tumour grafts and delayed-type hypersensitivity reactions is also discussed.