The effects of fish oil (FO) on immune complex nephritis induced by bovine serum albumin (BSA) were studied in female B10.Br mice. The mice were fed an experimental fat-free diet composed of either 10% FO, safflower oil (SO), or beef tallow (BT) as a lipid source throughout the study. Proteinuria was observed in 84% of the FO group (n = 19), 53% of the SO group (n = 19) and in 48% of the BT group (n = 19; p = 0.0217 vs. FO). The FO group showed a tendency toward more severe renal histologic changes than the SO and BT groups. The levels of anti-BSA antibody and circulating BSA-anti-BSA immune complexes were significantly higher in the FO group than in the SO and in the BT groups. Avidity of the anti-BSA antibodies showed a lower tendendy in the FO group. Prostaglandin E2 and thromboxane B2 productions by the renal cortex were much lower in the FO than in the other two groups. The ratios thromboxane B2/prostaglandin E2 were higher in the FO than in the BT group. These results suggest that FO oil supplementation leads to the deterioration of BSA-induced immune complex nephritis in mice due to the altered immune responses in association with suppressed prostanoid production.