The effect of hemoglobin on the ability of polymorphonuclear granulocytes (PMNs) to phagocytize and kill opsonized E. coli was measured. Results show that the addition of hemoglobin in a concentration of 0.1% inhibititis phagocytotic activity of PMNs from 68% to 13% after 30 minutes and from 73% to 45% after 60 minutes. The rate of intracellular killing after the addition of hemoglobin in the concentration of 0.5% declined from 63% to 21%. Though these experiments do not allow any conclusions as to the exact mechanism of action of hemoglobin in inhibition of phagocytosis, it seems likely from other data that the inhibitory activity resides in the heme part of the molecule. We conclude that hemoglobin inhibits phagocytotic activity and the ability to kill ingested bacteria of human polymorphonuclear granulocytes. This provides an additional mechanism for the adjuvant action of hemoglobin in intraperitoneal infections and supports the theory that hemoglobin acts directly on the granulocyte to impair the essential host defenses.