Complement receptor activity for cell bound C3b and C3d was detected on plasma membrane fragments prepared by nitrogen cavitation from cultured human lymphoid cells. The activity of the membrane fragments reflected the activity of the whole cells in that cells which did not form rosettes (P3J and RPMI 4098) resulted in inactive membranes and cells with high rosette formation (NC37 and Raji) yielded highly active membrane fragments. Two test systems were devised to detect these receptor activities, namely a rosette inhibition and a hemagglutination assay. Solubilization of C3 receptors was accomplished by extraction of active plasma membrane fragments with 2 MKBr. Dissociation and reassociation experiments suggest C3b and C3d receptors to be highly complex molecular structures. It appears that these complement receptors on plasma membranes rely on both protein and lipid moieties for the expression of their activity.