Postmortem changes in myoglobin concentrations in blood and organs were investigated using an enzyme immunoassay by animal experiments in combination with immunohistochemical staining of human cases. Blood myoglobin concentrations were found to increase drastically within a very short time after death. Those in striated muscle, however, did not change by day 14 postmortem. Myoglobin content in the liver and kidney increased slightly by day 5 postmortem, and more obviously by day 7 or later. However, almost no change was observed by day 5 in the kidney when the renal artery and vein had been ligated just after death. In the thyroid gland and the lung, the myoglobin content markedly increased by day 7 postmortem, with the logarithmical values rising nearly linearly as the time after death passed. In the thyroid gland, concentrations reached the level of the striated muscle. The mechanisms of postmortem myoglobin increase in organs are thought to be direct diffusion from the striated muscle and/or distribution through the blood. To estimate the postmortem interval, the determination of myoglobin content in the thyroid gland or the lung appears to be useful.