Heterophil phagocytosis of fluorescein-labeled staphylococcal bacteria was analyzed by flow cytometry. Opsonization with two types of normal pooled sera and staphylococcal antisera significantly increased bacterial phagocytosis compared to samples without an opsonin. The staphylococcal antisera did not significantly increase bacterial phagocytosis compared to the normal pooled sera. Opsonization appears to increase bacterial phagocytosis but specific antisera may not increase phagocytosis beyond that caused by pooled normal sera.