A comparison was made between the binding sites of two receptors that are believed to be closely associated on human B lymphocytes: complement receptor type two (CR2) that is specific for C3d fragments, and the receptor (EBVR) for Epstein Barr virus (EBV). Isolated fluid-phase CR2 bound to C3d on erythrocytes (EC3d) and inhibited both B cell-EC3d rosettes and the agglutination of EC3d by anti-C3d, it failed to inhibit either the binding or superinfection of B cells by EBV. By contrast, isolated fluid-phase EBVR inhibited EBV B cell binding activity and superinfection but had no CR2 activity. In addition, radiolabeled CR2 bound to EC3d and anti-CR2-Sepharose, whereas radiolabeled EBVR did not. Purified fluid-phase C3d fragments inhibited EC3d rosette formation with CR2+/EBVR+ cells but did not inhibit EBV binding. However, EBV binding to B cells did inhibit EC3d rosette formation. Clones of human/mouse somatic cell hybrids made from CR2+/EBVR+ human B lymphoblastoid cell and CR2-/EBVR- mouse myeloma cell parents expressed either EBVR or CR2 but only rarely expressed both EBVR and CR2. This suggested that the genes for EBVR and CR2 were located on two different human chromosomes. Thus it was concluded that CR2 is probably not the binding site for EBV.