A study is presented which compares the properties of antigens with relative mol. mass of about 40,000, detected by three different monoclonal antibodies: IV.3, 41H.16 and KB61. Antibody IV.3 has been shown (in work by other investigators) to detect the Fc receptor for IgG. It recognizes this molecule on neutrophils, macrophages and platelets. Antibody 41H.16 precipitates a molecule of mol. wt of approximately 40,000, which is present on B cells, neutrophils and macrophages. The antigens precipitated by IV.3 and 41H.16 have been compared by isoelectric focusing, sequential antibody-mediated affinity chromatography and trypsin hydrolysis. The results indicate that these two antibodies bind different molecules of similar relative mol. mass. A third antibody, KB61, has been reported, which also reacts with a 40,000 mol. wt molecule and has a distribution of cellular reactivity which is similar to 41H.16. Sequential preclearing experiments have shown that these molecules recognize the same antigen.