Authors have observed that the shellfish from Minamata Bay ashed by heat was deprived of its toxicity to animals, so the causative agent of the Minamata disease was considered to be an organic compound. And then an alkyl mercury compound was proved in the fish and shellfish from the bay. It was proved experimentally that the alkyl mercury compound in the fish and shellfish was not synthesized from the inorganic mercury compounds accumulated in the mud of the drainage from Minamata Factory and the bay, and an alkyl mercury compound discharged in the bay had directly accumulated in the fish and shellfish. On the other hand, the waste treatment plant was completed at the beginning of 1960. Since then the mercury content in the fish and shellfish from the bay had gradually diminished, but in January of 1961 an organic mercury compound which was distilled with steam was proved from the mud in the drainage from the factory, but the organic mercury compound had diminished from the mud relatively in short time. The organic mercury compound proved from the mud of the drainage was considered that it was discharged from the factory, for the mercury content in the shellfish from the bay had temporarily increased at the same time.<br>Then methyl mercury chloride (CH₃HgCl) was extracted from the sludge accumulated in the reaction tube of the acetaldehyde plant of the factory and the same compound was extracted from the shellfish of the bay. Methyl mercury chloride (m.p. 174°C) have IR absorption spectra at 1191cm^-1 and 788cm^-1 and stable in acidic and alkaline solution and distilled with steam in acidic solution, and have low Rf value in the paperchromatography.<br>Various methyl mercury compounds such as bis (methyl mercury) sulfide [(CH₃Hg)₂S], methyl (methylthio) mercury (CH₃HgSCH₃), methyl mercury hydroxide (CH₃HgOH) and so on were synthesized for the purpose to know whether methyl mercury chloride discharged from the factory might have changed to another methyl mercury compound or not. But experimental results showed that methyl mercury chloride discharged from the factory had directly accumulated in the fish and shellfish of the bay.<br>Further experiments proved that symptoms and findings of cats administered per os with the above described methyl mercury compounds were the same with those of animals administered with the fish and shellfish from the bay.<br>From the above results it was concluded that the causative compound of the Minamata disease was methyl mercury chloride and the active agent for the development of the disease was methyl mercury radical (CH₃Hg-).