Natural killer (NK) cells that infiltrated into the primary tumour site at an early stage of tumour development, were examined for their participation in the generation of anti-tumour cytotoxic T lymphocytes (CTL). NK cells, which were detected by anti-NK1.1 monoclonal antibody (mAb), increased in the peritoneal exudate cells (PEC) on days 3 and 7 after an intraperitoneal (i.p.) inoculation of syngeneic B16 melanoma cells. These tumour-infiltrating NK cells showed a high level of cytotoxic activity against NK-sensitive YAC-1 cells and an increased expression of interferon-gamma (IFN-gamma) mRNA and interleukin-2 (IL-2) mRNA. The in vivo depletion of NK cells with anti-NK1.1 mAb, prior to i.p. inoculation of B16 melanoma cells, resulted in an increased number of tumour cells in the PEC compared to NK cell non-depleted mice. Interestingly, the differences in tumour cell number between both groups were more prominent on days 7 and 14 than on day 3, which strongly suggested that early-infiltrating NK cells have a large influence on the subsequent anti-tumour response. The in vivo depletion of NK cells prior to immunization with melanoma cells abrogated the capacity of the spleen cells to generate CD8+ tumour-specific CTL after in vitro restimulation. This inability of generating anti-tumour CTL was partially restored by additional i.p. injections of recombinant IL-2 and/or IFN-gamma simultaneously with the immunization of melanoma cells. The in vitro depletion of NK cells prior to the in vitro restimulation with melanoma cells partially impaired the anti-tumour CTL generation from the spleen cells of the immunized mice. Lastly, the in vivo depletion of NK cells prior to immunization with melanoma cells abolished the protective immunity against melanoma cells at the rechallenge. Overall, these results indicate that early-appearing tumour-infiltrating NK cells not only participate in the anti-tumour early defence by themselves, but also play a crucial role in the generation of anti-tumour CTL.