BL13, a mouse monoclonal IgG1 antibody raised against human B cells, blocked the function of the C3d receptor (CR2) and bound with high affinity (5 X 10(8) L M-1) to CR2 on B lymphoma cells. Following capping with the second antibody, BL13 inhibited C3d-dependent rosette formation of Daudi and Raji cells and C3b-dependent CR2-mediated rosette formation with B lymphoma cells, but did not inhibit CR1-mediated rosettes between C3b-bearing cells and peripheral blood lymphocytes. Competitive binding experiments between biotinylated BL13 or anti-CR2 antibody HB-5 and unlabelled antibodies demonstrated that BL13 bound to an epitope that is distinct from that recognized by HB-5, and closely associated with that recognized by monoclonal antibody anti-B2. BL13 only reacted with some B cells and follicular dendritic cells in germinal centres in human lymph nodes, whereas HB-5 strongly reacted with circulating B cells and bound to most cells in the follicles. These results demonstrate the heterogeneity of antigenically defined CR2.