We have compared and contrasted the binding of Agg IgM and heavily sensitized EAB (IgM) complexes to the Fc mu receptor of normal and neoplastic human lymphocytes. Agg IgM binds uniformly to the entire SmIg+ B cell population yet normal lymphocytes require culture in order to achieve binding of EAB complexes to a subset of SmIg+ B cells. In blocking studies IgM complexes and IgM aggregates appear to detect the same receptor and with both reagents binding is influenced by the presence of Mg2+ but not Ca2+ and is inhibited by EDTA. The percentage of cells binding EAB was highest in normal B lymphocyte fractions enriched for C2+ cells (CRL+). EAB binding to cells in the CRL- fractions was negligible even though CRL- fractions contained cells which were SmIg+C-3. EAB bound only to neoplastic chronic lymphocytic leukemia cells (CLL) that expressed a high percentage of C+3 cells. Clones lacking a C3 receptor failed to bind EAB. Thus, the binding of EAB complexes to B lymphocytes appears to be associated principally with a subset that express a C3 receptor whereas IgM aggregates bind to the entire SmIg+ B cell population.