After opsonization with whole human serum, with IgG antibody alone or with C3 fragments alone, the ingestion of zymosan particles by human neutrophils was found to correlate most closely with the efficiency of C3 fragment deposition on the zymosan surface. Selective opsonization of zymosan particles with IgG did not promote phagocytosis at all in suspension, but could promote ingestion after particle-cell contact was induced by centrifugation. In contrast, zymosan particles in suspension selectively opsonized by C3 fragments were ingested as efficiently as those opsonized with whole serum, and we suggest that C3 fragments provide the principal stimulus for phagocytosis of zymosan. The process of vacuolar sealing was not dependent on the state of opsonization of the particles and was significantly more efficient with unopsonized particles, indicating that formation of unsealed vacuoles is not due to a failure of the 'zipper' mechanism of sequential interaction of cell surface receptors and opsonic ligands.