In this study, we examined the effects of chronic sublethal low pH exposure (pH 4.5) on endocrine and immune functions in carp. Plasma cortisol levels elevated rapidly from 3 hours after acid exposure and kept high until 2 days. Subsequently cortisol levels decreased gradually, but were significantly higher than those of control groups even at 4 weeks after acid exposure. Plasma immunoglobulin M (IgM) levels declined transiently at 1 week after the initiation of acid exposure, and then recovered to control levels at 4 weeks after acid exposure. Phagocytic rates of peripheral blood leukocytes also decreased significantly after 1 week of acid exposure, and was still low at 4 weeks. These results suggest that acidification stress causes a transient depression of the non-specific immune activity by the immunosuppressive effect of cortisol.