Epoxidised soya bean oil (ESBO, 1000, 2000 or 4000 mg/kg) was orally administered to BALB/c mice daily for 28 consecutive days, and the control mice were exposed to vehicle (corn oil). Mice were immunized and challenged with sheep red blood cells (SRBC) or bovine serum albumin (BSA). In groups exposed to ESBO, the body weight gains and the relative lymphoid organ weights were not significantly changed as compared with control group. Secondary IgG antibody response to BSA was not significantly changed by ESBO, but plaque-forming cell (PFC) response to SRBC was significantly suppressed in mice treated with 4000 mg ESBO/kg/day. The mitogenic response of splenic B cells induced by LPS was not effected by ESBO in any of the groups. These results indicate that ESBO did not induce significant humoral immune response at a dose less than 2000 mg/kg/day in mice.