Immunoglobulin (Ig) bearing, antigen-binding and antibody-secreting cells were investigated for the presence of membrane receptors for the Fc part of IgG and for C3b on their surface. Fc and C3b receptors were detected on the surface of most Ig-bearing and of most antigen-binding cells. Fc receptors were also detected on IgM antibody-secreting cells but not on IgG-secreting cells. C3b receptors were not detected on any antibody-secreting cells. These receptors are either lost from the B cell surface as they differentiate into antibody-secreting cells or are blocked in vivo by immune complexes or C3b.