Native agarose beads (diameter 5-10 micron), activators of the alternative complement pathway, are slowly phagocytosed when incubated with human monocytes cultured under serum-free conditions. Agarose beads preincubated with monocyte cultures and then transferred to new cultures are more easily phagocytosed than native beads. These results indicate that the phagocytosis of agarose beads depends on opsonization of the beads by one or several substances of monocyte origin. By using antihuman C3 antibodies, trypsin treatment, and sodium dodecyl sulphate washing, we were able to demonstrate C3b and iC3b on the agarose beads. The molecules were covalently bound to the surface of the beads. We conclude that in vitro human monocytes produce and secrete the essential factors for activation and propagation of the alternative complement pathway (factors C3, B, D, H and I), which becomes evident with an external activator like agarose beads in the cultures. The activation of complement by agarose beads results in the attachment of C3b and iC3b to the surface of the beads, which are then phagocytosed by means of C3b and iC3b receptors on the monocytes.