Circulating neutrophils isolated from patients 3-4 h after a myocardial infarction produced less O2-. compared with controls, when stimulated with phorbol myristate acetate or formyl-methionine-leucine-phenylalanine. Three days after the infarction the O2-. generation elicited by both stimuli further decreased markedly. Seven and 15 days after infarction the O2-. stimulated production was only slightly lower than or similar to the control values. The neutrophils of infarcted patients showed an augmented latency period before O2-. production compared with controls in response to exogenous stimuli, particularly three days after infarction. Electron microscopy revealed that the neutrophils isolated from the infarcted patients displayed signs of cell exhaustion with few alterations of the plasma membranes when stimulated with phorbol ester. In contrast, control neutrophils displayed alterations of the plasma membranes characteristic of active neutrophils. The results of this study indicate that the circulating neutrophils appear exhausted and functionally inhibited immediately after myocardial infarction.