It has been known that sea mammals accumulate a large amount of mercury (Hg) together with Selenium (Se) in the liver, but they look like healthy. Therefore, Hg may be detoxified by a certain way. We were interested in the chemical form of Hg. We thought that Hg directly binds to Se to be a nontoxic Hg/Se-containing compound. So, we attempted to solubilize an Hg-containing compound from striped dolphin (Stenella coeruleoalba) liver with various kinds of solvents, and an SDS-containing buffer solution was most effective. The extract was subjected to various purification methods, but in spite of many efforts, Hg-containing compound could not be isolated. On the other hand, the residue after defatting with acetone followed by extraction with 0.2 M ammonium acetate was subjected to TEM analysis, resulting in the discovery of many small crystalline materials. Analyses of these crystals revealed that they were solid solution, Hg(Se0.66S0.34), which is completely insoluble in ordinary solvents and thus is thought to be the detoxified form.