During the investigation of insulin-like effects of low molecular weight thiols in rat adipocytes it was found that thiol autoxidation to H2O2 occurred even in the absence of albumin and cells. Since chromatography of the thiols on a chelating ion exchange resin prevented H2O2 formation, it is likely that trace metal contaminations of the thiol preparations catalyzed thiol autoxidation in oxygenated solutions. Incubation of rat adipocytes with 1 mM dithioerythritol and low concentrations of dialyzed albumin stimulated 2-deoxyglucose transport, glucose metabolism to CO2 and lipids, and formate resin or in most instances incubation with EDTA or catalase prevented these effects. The antilipolytic effect of dithioerythritol was also abolished by catalase. The insulin-like effects of thiols therefore appear to be solely mediated by H2O2 generated during thiol autoxidation catalyzed by trace metal contaminants either of albumin or of the commercial thiol preparation itself.