This study determined the effect of the alternative complement pathway (ACP) on neutrophil and monocyte phagocytes from peripheral blood of channel catfish, Ictalurus punctatus. Fluorescent-labeled latex microspheres, Edwardsiella ictaluri and Escherichia coli were used to quantify phagocytic attachment and ingestion. Activation of the ACP enhanced the attachment of bacteria and microspheres to neutrophils and monocytes. Activation of the ACP by serum opsonization of Escherichia coli increased its ingestion by neutrophils in comparison with Edwardsiella ictaluri and microspheres. Inactivation of the ACP and C3b diminished attachment by neutrophils and monocytes, and ingestion by neutrophils of microspheres and bacteria. Ingestion by monocytes was not affected by activation of the ACP. In the present study, we found that the ACP functions in opsonophagocytosis in catfish and that ingestion by neutrophils was especially enhanced.