We have compared the effects of dietary saturated and unsaturated fats of the n-6 and n-3 types on the immune status of male New Zealand white rabbits. Four groups of rabbits (n = 8) were fed purified diets containing one of the following fats (7.6% w/w, 23 kcal%) for 5 mo, hydrogenated soybean oil (HSO); safflower oil (SFO); linseed oil (LSO); or menhaden oil (MHO). In vitro proliferation of peripheral blood lymphocytes (PBL) cultured with T-cell mitogens was significantly higher in the LSO group than in the other three groups, and that in the HSO group was higher than in the MHO and SFO groups which were not different from each other. Proliferation of PBL in response to B-cell mitogens was significantly higher in the LSO group than in the SFO and MHO groups. In vitro proliferation of splenocytes (SPC) from the LSO group was higher than that from the other three groups only when SPC were cultured with T-cell mitogens. Serum antibody levels against bovine serum albumin were significantly higher in the LSO group than in the SFO group after second and third immunizations. Spleen weights, number of SPC or PBL, and delayed-type hypersensitivity were not different among the four dietary groups. Our data show that linseed oil diet enhanced several indices of immune status in rabbits.