Dietary long-chain (n-3) fatty acids from fish oil and low intensity exercise have been reported, independently, to inhibit tumor growth in rats. The mechanism for these effects is not known but may be related to diet and exercise-induced alterations in immune function. To study the individual and combined effects of these interventions on anticancer immune responses, healthy Fischer 344 rats were fed, for 4 wk, one of two semi-purified diets (polyunsaturated to saturated fatty acid ratio = 0.9), which differed only in the composition of fat (200 g/kg) and provided long-chain (n-3) fatty acids at 0 or 33 g/kg of total fat. Rats were randomly assigned to groups in a 2 x 2 experimental design to swim 3 h/d or to remain sedentary. For sedentary rats, dietary (n-3) fatty acids increased (P < 0.05) splenic natural killer (NK) cell cytotoxicity and the percentage of activated (CD71+) T and B cells and macrophages in spleen after concanavalin A stimulation. For exercise-trained rats, feeding the high (n-3) diet decreased (P < 0.05) the percentage of CD71+ T helper and B cells after stimulation. NK cell cytotoxicity, and the percentages of CD71+ T cells, B cells and macrophages after stimulation in the high (n-3)-fed exercise-trained group were not different than those of the low (n-3)-fed sedentary group. Thus individually, but not in combination, long-chain (n-3) fatty acids and low intensity exercise may be advantageous by augmenting cell-mediated immune function and NK cell cytotoxicity in healthy rats.