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C3 was bound to human erythrocytes from autologous plasma or from serum brought to low ionic strength (mu less than or equal to 0.03) and pH between 4.0 and 5.0, then subsequently incubated with erythrocytes (50/1, v/v) for 20 min at 0 degree C. This capacity was preserved up to 72 h by prolonged incubation at 20, 25 or 37 degrees C, whereas it was quickly lost by incubation at 0 degree C. C3 binding did not require complement activation and was not observed with neuraminidase-treated erythrocytes. Crossed immunoelectrophoretic analysis of the pretreated serum or plasma revealed that a fraction having more cathodal migration than that of native C3 was generated upon incubation in the above-mentioned conditions. This fraction appeared able to selectively bind to the erythrocytes. Cell-bound C3 reacted positively to antisera against C3a, C3c, C3d or native C3; they rosetted positively with EAC3b, clearly showing that this C3 binding was not dependent on the proteolysis of C3 and that it concerned the acceptor sites on the cells, since C3b receptors were free. The functional significance of this C3 binding was also investigated: EC3 were not able to lyse through the alternative pathway, whereas lysis clearly increased when C3 was found to AET-treated erythrocytes. This finding, together with the modulation in the capacity of EC3 or E(AET)C3 to form an alternative pathway convertase by antibodies to C3c or C3d, strongly suggests a contribution of bound C3 to such a convertase. In contrast to "C3b-like" C3, this modified C3 was able to bind to acceptor sites on erythrocytes but, like the former, it retained the capacity to form an alternative pathway convertase. In this light, it may represent an intermediate between C3 and "C3b-like" C3.