The CD24 human B cell-associated antigen was originally characterized in this laboratory with the use of monoclonal antibody BA-1 combined with a standard radioimmuneprecipitation technique, and was reported to be a three chain glycoprotein complex of 45, 55, and 65 kilodaltons. We have compared our standard radioimmuneprecipitation technique (BA-1 ascites followed by rabbit anti-mouse IgM-coated protein A-Sepharose) to BA-1 conjugated directly to CNBr-Sepharose (BA-1-Sepharose) and report striking differences in the electrophoretic profile of CD24 immuneprecipitates. The CD24 immuneprecipitate obtained with BA-1-Sepharose showed a single broad band with a relative mobility of 42 kilodaltons. A series of experiments performed with the two immuneprecipitation techniques, reducing or nonreducing electrophoretic conditions, and addition of preformed mock BA-1 immuneprecipitate to BA-1-Sepharose immuneprecipitates convincingly demonstrated that the previously described 55 and 65 kilodalton components were artifacts caused by co-migration of CD24 with IgG and IgM heavy chains, respectively. Because of the consistent association and co-migration of CD24 with IgG heavy chains, we investigated the possibility that CD24 might be related to the 45 kilodalton Fc-gamma receptor expressed on B cells and eosinophils. We found, however, no evidence for such a relationship by cross adsorption analysis with BA-1-Sepharose and IgG-Sepharose.