Microscopic observation, using physical development of silver, was carried out to localize the mercury-selenium interaction products in the organs of Mediterranean striped dolphins. The silver-metal reaction products were located mainly in hepatocytes and macrophages for liver, in proximal tubules for kidney. They were less abundant in lung than in liver and kidney. The result of semi-quantitative histochemistry tests showed that silver staining deposits were more abundant at relatively high metal concentrations than low metal contents, but independent of the metal contents. Comparisons with the most concentrated metal contents suggested that there might be a new complex of mercury and selenium, which could not be stainable by physical silver development.