From clinical and pathological point of view, the causative agent of Minamata disease has been supposed to be an organic mercury compound and abnormally high content of mercury was detected in the fish and shellfish from Minamata Bay and in organs of cases of the disease. Then, what kind of mercury compound is contained in the fish and shellfish? In this paper, the biological and chemical properties of the toxic substance in the fish and shellfish was studied. The results were as follows:<br>1) The mercury in the shellfish from Minamata Bay, as it is, can not be extracted with water and organic solvents. But one third of the mercury in the raw shellfish is extracted with 3% NaCl solution at low temperature (2°C). The mercury in the NaCl solution is precipitated gradually with protein by salting out and dialysis, and all the mercury in the solution is precipitated with protein by heating at 63°C. It was proved that all the mercury in the shellfish was combined with the protein of the shellfish, including the mercury combined with simple protein, such as globulin and albumin.<br>2) From hydrolytes of the shellfish with alkali and acid, mercury is extracted with organic solvents. In the brain of rats administered with the extracted mercury, abnormal amount of mercury was observed, so the extracted mercury compound seems to be an organic form.<br>3) When the dry shellfish powder is digested with 0.04% pepsin solution of pH 1.6 at 37°C for 24 hours, one third of the mercury is tranferred to the digested fluid. When the extracted protein from the raw shellfish and the fish powder from Minamata Bay are digested with the same method, most of the mercury in the protein is transferred to the digested fluid.<br>4) By steamdistillation of these digests, almost all the mercury in the digests is transferred to distillates. The mercury in distillates was extracted with organic solvents. A cat was administered with the mercury compound in the distillate from the digest of the shellfish, and the abnormal mercury retention in its brain was observed.<br>5) But the mercury in the digest is not distilled at the temperature below 100°C with normal atmospheric pressure. And when pH of the digest is adjusted to neutral or alkali, no mercury is distilled by steam. No mercury is distilled by steam from the dry shellfish powder, as it is, or inorganic mercury compounds in acid solution.<br>6) The liver of a human autopsy case of Minamata disease and of a cat attached by the disease through feeding of abundant fish and shellfish from Minamata Bay, was digested by pepsin, and the digests were distilled by steam. Then a part of the mercury in the livers was transferred into the digests and the mercury in the digests was distilled by steam, and the mercury in the distillates was extracted with organic solvents. It was proved that in organs of human and animal cases of Minamata disease an organic mercury was accumulated.<br>7) The properties of the organic mercury compounds in the shellfish from Minamata Bay were compared with those of known alkyl mercury compounds. From steam distillation, solubility and stability in NaHSO₃ and NaHCO₃ solution, Rf value of paperchromatography and behavior towards ion exchange resins, the organic mercury compound in the shellfish is considered to be an alkylmercury compound, in the molecule of which has carbonyl radical or double bond.