We have obtained a homogeneous preparation of a beef liver thiol: protein disulfide oxidoreductase which degrades insulin and other peptides and proteins by disulfide bond cleavage. Presently we are characterizing this enzyme in terms of its molecular weight, subunit composition and other physiochemical characteristics. The proposed research is to continue our investigation into the mechanism of action of the thiol:protein disulfide oxidoreductase. This includes both active site and kinetic studies.