An immunoradiometric assay (IRMA) for the quantitative analysis of human chorionic gonadotropin (hCG) was evaluated for specificity, sensitivity, accuracy and precision. The results were compared with those of the conventional radioimmunoassay (RIA) used in our laboratory. The IRMA is a solid-phase, double-antibody immunoassay that sandwiches the intact hCG molecule between the two antibodies. It has specificity, accuracy, and precision which are similar to those of the RIA. The RIA is based upon the assumptions that the antigenicity of the tracer is not altered by the iodination process and that the antibody reacts equally with all of the antigens, including the radiolabeled antigen. The IRMA does not use radiolabeled antigens and thus is free of the assumptions made in the conventional RIA. The IRMA may be more accurate at the lower limits of the assay because it does not require logarithmic transformations. Since the IRMA does not measure the free beta-subunit of hCG, it cannot be endorsed as the sole technique to quantitate hCG in patients with gestational trophoblastic neoplasia until the significance of the free beta-subunit in these patients is determined.