The cytotoxic effects mediated by lymphocytes from cancer patients after in vitro "education" on autologous tumor cells have been investigated. Peripheral blood lymphocytes from three sarcoma patients were cultivated on autologous tumor-cell monolayers and tested thereafter in a micro-cytotoxicity assay against tumor and fibroblast cells. This procedure led to the progeny of non-specific killer cells. As the phenomenon did not occur when the same lymphocytes were co-cultivated with autologous fibroblasts, the generation of non-specific effector cells may have been caused by specific antigenic triggering. The presence of autologous serum during "education" was found to inhibit the manifestation and/or the generation of killer cells. The same serum was without effect when added during the cytotoxicity assay only.