A study was undertaken to reveal the role of Fc and C3b receptor of mouse peritoneal macrophages (MPM) in the uptake of radiolabelled immune complexes. Large latticed preformed complexes consisting of human serum albumin (HSA)-anti HSA at equivalence (IC-Eq) and with antibody excess (IC-Ab) were observed to be avidly taken up by resident macrophages unlike small size complexes with antigen excess (IC-Ag). Macrophages elicited by thioglycollate (Tg) showed higher IC-binding capacity while IC-elicited MPM showed reduction in the same when compared to the resident cells. However, complement coated complexes were significantly taken up by these IC-elicited macrophages. Uptake studies were further extended to determine the expression of Fc and C3b receptor activity in MPM when elicited with preformed IC. Tg-elicited MPM were observed to bind greater number of IgG-coated erythrocytes (E-IgG) than resident MPM whereas IC-elicited MPM bound E-IgG poorly. When Fc receptors were blocked by in vitro IC treatment, poor binding of complement coated E-IgG [E(IgG)C] was recorded in resident MPM. The present complement medicated rosetting data tends to show enhanced expression of C3b receptors on IC-elicited macrophages.