The microheterogeneity of plasma albumin, originally proposed as a means of explaining the electrophoretic separation of rapidly interconverting conformers, has been supported by a variety of evidence. Plasma albumin is known to consist primarily of mercaptalbumin, with significant amounts of two nonmercaptalbumin components. It was therefore of interest to prepare the mercaptalbumin component in the purest form possible and ascertain whether or not it is heterogeneous. A new method for the preparation of defatted mercaptalbumin by ion-exchange chromatography is described.Evidence is presented that this mercaptalbumin preparation, made from either crystallized or noncrystallized bovine plasma albumin, is virtually free of lipid-bound albumin, dimer, or oligomers of albumin, and of any forms with abnormal disulfide pairing such as are believed to arise on aging albumin in solution. The mercaptalbumin is more homogeneous than the parent plasma albumin as judged by the Q k^/ogami and Foster (1963) concluded that plasma albumin is microheterogeneous, i.e., it consists of a large number of closely related species. They based their conclusion principally on two observations: N and F forms, although rapidly interconverting, were electrophoretically resolvable, and the apparent order in H+ ions of the N-F transition was lower than expected on the basis of titration data. Other evidence in support of albumin heterogeneity has been the fractionation of albumin by partial precipitation in concentrated KC1 solution (Petersen and Foster, 1965a;Habeeb, 1968;McMenamy and Lee, 1967) and by partial denaturation (Stokrová and Sponar, 1963) into fractions differing in titration behavior, disulfide reducibility, and stability toward denaturation.The most widely used criterion of albumin heterogeneity has been its solubility-pH profile in 3.0 m KC1, a method developed by Petersen and Foster (1965a,b), and later modified by Sogami and Foster (1968). It has been shown that more heterogeneous albumin preparations have broader solubility-pH profiles, i.e., larger values of 90 as defined by Sogami etal. (1969).Recent investigations of microheterogeneity have placed considerable emphasis on purity of the albumin preparation, specifically on removal of nonmercaptalbumins and bound lipid impurities. Foster (1967) andFoster (1967) pointed out the large decrease in 90 observed upon de-
