We studied the phagocytosis of agarose beads by human alveolar macrophages in terms of the morphology, the receptors involved, and the cellular substrates (plastic or fibronectin) used. Beads coated with C3b (58%) and iC3b (42%) by treatment with serum, were ingested during 45 min by CR1 and CR3 on the macrophages. This ingestion was inhibited 80-90% by the presence of polyclonal F(ab')2 anti-C3 fragments. Since the phagocytosis of both C3b- and iC3b-coated beads was about threefold stronger than for C3b-coated beads (trypsinized serum-treated beads), the results indicate that the CR3 is more phagocytic than the CR1. The phagocytosis of initially complement uncoated beads, which are slowly opsonized with macrophage-produced C3b and iC3b in vitro, was also strongly inhibited (70-80%) by the presence of anti-human C3 F(ab')2 fragments. There was an increased phagocytosis (10-17%) of complement precoated beads by macrophages cultured on the fibronectin substrate versus the plastic substrate. The morphology and rapid phagocytosis of the complement precoated beads was demonstrated by SEM. The general impression was that membranous protrusions stretched towards the beads, which became increasingly enclosed by plasma membrane.