The effect of anesthesia and major abdominal surgery on zymosan-induced chemiluminescence (CL) of neutrophil granulocytes was evaluated. CL was measured in diluted whole blood taken at distinct intervals within the perioperative period. In addition, blood samples from healthy volunteers were supplemented with ether and halothane to investigate the in vitro effect of these agents. The phagocytosis-induced CL was not found to be depressed by anesthesia and surgery. Only at supranarcotic concentrations was CL reduced. Surgery and anesthesia, therefore, do not appear to impair this defense system significantly under the conditions of this investigation.