Accurate assays for IgG subclasses are needed for a variety of studies underway in the Plasma Derivatives Laboratory, including studies on the fragmentation of IgG, the antibody specificity of various subclasses, and the relative propensity of the different subclasses to dimerize. In addition, analysis of the subclass distribution of Immune Globulin Intravenous (Human) from different manufacturers can be valuable in comparing different manufacturing methods. Two assays have been utilized. For samples of Immune Globulin and Immune Globulin Intravenous, radial immunodiffusion (RID) with commercially available plates proved to be reliable, accurate and simple to perform. RID is rather expensive and requires several days for completion, however. The second assay that has been developed is a direct ELISA in which sample is adsorbed to wells of a microtiter plate, then assayed by means of alkaline phosphataseconjugated mouse anti-subclass monoclonal antibodies. This ELISA method has much greater sensitivity than the RID assay and is less expensive, but appears to be somewhat less accurate. It has been used primarily in the assay of material obtained during attempts to define procedures for the separation of the four IgG subclasses.