By treating C3 with purified C1, C4 and C2 in the fluid phase, haemolytically inactive C3 was prepared. This was shown to bind to human erythrocytes by use of radio-labelled (Fab')2 antibody to guinea-pig C3. The activated C3 preparation inhibited immune adherence between EAC43 and human erythrocytes. These findings indicate that the activated C3 attaches to the immune adherence receptor on human erythrocytes. In addition the fluid-phase activated C3 adhered to thymus cells and sheep erythrocytes, whereas EAC43 did not. Thus the immune adherence receptor may be present on so called immune adherence-negative cells, but in insufficient concentration to form rosettes with EAC43.