An arginine-glycine-aspartic acid sequence (RGD in the single letter code for amino acids) is present in the cell attachment site of both vitronectin and fibronectin. Inasmuch as fibronectin and synthetic peptides containing RGD enhance ingestion of opsonized particles by monocytes, we investigated the effects of vitronectin on phagocytosis by monocytes of sheep erythrocytes bearing IgG (EA) or complement C3b (EC3b). Peripheral blood monocytes were isolated by countercurrent elutriation and allowed to adhere to slides that had been coated with either vitronectin or fibronectin. Next, EA or EC3b were incubated with the adherent monocytes, and phagocytosis was subsequently quantified. Vitronectin caused the same dose dependent increase in phagocytosis as fibronectin. The augmentation of phagocytosis of EA induced by vitronectin could be inhibited by the F(ab')2 fragments of anti- vitronectin IgG but not by preimmune F(ab')2. The maximum phagocytosis of EA induced by vitronectin could not be enhanced by the addition of fibronectin, suggesting that vitronectin and fibronectin act on the same population of monocytes and that the two proteins stimulate the same mechanism through which the enhanced phagocytosis is mediated. Fibronectin and vitronectin caused a tenfold increase in the attachment of EC3b to monocytes, but phagocytosis was augmented minimally. These studies demonstrate that vitronectin modulates interactions between monocytes and opsonized particles.