The presence of receptors on the plasma membrane of human polymorphonuclear (PMN) leucocytes for factors related to complement and for the Fc region of immunoglobulin has not been clearly defined for opsonized bacteria. To separate the activity of these two receptors, the uptake of [3H]thymidine labelled staphylococci opsonized with normal serum or heat-inactivated serum was measured. Phagocytosis was depressed when bacteria opsonized with normal serum were incubated with trypsin-treated leucocytes, suggesting that complement receptors of human PMN leucocytes are trypsin-sensitive. Phagocytosis of bacteria opsonized with heat-inactivated serum was not depressed by trypsin, but was blocked by incubating PMN leucocytes with heat-aggregated IgG and by incubating opsonized bacteria with protein A. In experiments performed to quantify the number of bacteria attached to but not ingested by PMN leucocytes, it was shown that both complement and Fc receptors participate in the ingestion phase of phagocytosis. Cell membranes of human PMN leucocytes possess two receptors for opsonized staphylococci; a complement receptor which is utilized when bacteria are opsonized in normal serum and an Fc receptor when bacteria are opsonized in heat-inactivated serum. Both receptors participate in the ingestion as well as the attachment phase of phagocytosis.