ABSTRACT: Immunoblotting has been utilized to detect the presence of human chorionic gonadotropin (hCG), its α and β subunits, the Tamm-Horsfall protein (uromodulin), immunoglobulins G (IgG) and M (IgM), kappa (K) and lambda (L) chains, and serum albumin in commercially available preparations of hCG intended for human use. Concentrated pregnancy, postpartum, and normal urines were studied as a comparison. Those hCG batches prepared from pooled first-trimester pregnancy urine contained all of the hCG and non-hCG proteins listed above or their fragments, with the single exception of the IgM μ chain. The conflicting literature regarding the immunomodulatory properties of hCG requires reevaluation, since many previous reports of immunologic activity utilized these preparations, containing intact and degraded IgG, K, and L chains, and the Tamm-Horsfall protein, all of which may contribute to an altered immune response. Since hCG can be prepared in high yields, free of these contaminants, these data suggest that patients are being unnecessarily exposed to contaminating substances when receiving parenteral injections of hCG.