(C57BL/6 x DBA/2)F1 mice were fed antioxidant-matched fish oil (FO) or corn oil (CO) diets for weeks, were challenged with B16.F10 melanoma cells, and lung metastases were enumerated 17 days later. Mice fed FO had fewer lung tumors than mice fed CO. This dietary effect persisted in mice injected with monoclonal antibodies which depleted natural killer cells, CD8+ T cells or CD4+ T cells. Generation of specific anti-B16.F10 cytotoxic cells in vitro by splenocytes from immunized mice was greater in mice fed FO. Even though host resistance to lung metastases by B16.F10 cells is mediated in large part by natural killer cells, CO and/or FO may have affected cells not tested here or tumor cells themselves.