Human medium-chain enoyl-CoA hydratase was purified from liver, because we noticed the presence of a high medium-chain enoyl-CoA hydratase activity in human skin fibroblasts catalyzed by an enzyme different from the known enzymes catalyzing the enoyl-CoA hydratase reaction. Two enzyme preparations were obtained. One of them, preparation I, consisted of 46-kDa polypeptide, and its molecular mass was estimated to be 86 kDa. The other, preparation II, consisted of a major 77-kDa polypeptide and minor smaller polypeptides including 46-kDa polypeptide. The molecular mass of preparation II was 154 kDa. Both enzyme preparations catalyzed reversible dehydration of medium-chain D-3-hydroxyacyl-CoA to 2-trans-enoyl-CoA, but did not react with L-3-hydroxyacyl-CoA. Catalytic properties and immunochemical reactivities of these enzyme preparations were nearly the same. The cross-reactive material to the antibody was confirmed to be in peroxisomes by immunohistochemical study of cultured human skin fibroblasts.