Effect of organic mercury on the immune system was studied in experimental acute methyl mercury intoxication in mice. After 7 daily injections of methyl mercury hydroxide, marked atrophy of the thymic cortex and splenic lymphoid follicles were observed, associated with decrease of PHA‐ and LPS‐responsiveness of splenic lymphocytes as well as conversely increased both PHA‐ and LPS‐responsiveness of thymocytes. In addition, apparent decrease of antibody formation against sheep red blood cells was observed at 7 day after the last injection. However, all these changes restored to normal level within 4 weeks after the last injection. Histochemically, mercury deposits were massively found in the renal tubules and slightly in macrophages throughout the body at 7 day after the last injection. The mercury deposits in macrophages of the thymus, lymph nodes and liver increased in concentration thereafter, with concomitant decrease of those in renal tubules. Presence of mercury element in these deposits was confirmed by electron probe analysis. It was suggested that the organic mercury gave direct cytotoxic effect on lymphoctes, but the effect appeared to be soon detoxicated by macr