The availability of techniques such as surgical reperfusion, angioplasty, and thrombolysis for the treatment of acute myocardial infarction (AMI) has revived interest in seeking an early detectable biochemical marker diagnostic for AMI. Therefore, we investigated whether an unidentified oxidase that is released by activated neutrophils at the onset of AMI could be used as an early diagnostic assay. The conversion by plasma oxidase of 1 μM of adrenaline to 1 μM of adrenochrome represents the plasma oxidase activity (POA) of 1 U/L. Fifty patients suspected of having AMI, 40% of whose electrocardiograms were nondiagnostic for AMI, were admitted to the coronary care unit, and venous blood samples were obtained for determination of the POA and creatine phosphokinase-MB levels. Healthy volunteers (n= 12) served as control subjects, and 8 patients with pneumonia whose leukocyte counts were greater than 15,000 μL were included in the study. In those with AMI (n= 22), as determined by serial creatine phosphokinase-MB, the mean POA (± standard error of the mean) was 233 ± 13 U/L, and in those with angina and no AMI (n= 28) was 127 ± 5 U/L (P< 0.0001). In the control group, mean POA (± standard error of the mean) was 84 ± 5 U/L (control versus angina;P< 0.01) and for those with infection was 214 ± 10 U/L. At admission, the creatine phosphokinase-MB was diagnostic for only 12 of the 22 patients with AMI (sensitivity rate of 54%), whereas in 21 of those patients, the POA values were diagnostic for AMI (sensitivity rate of 95%). Determination of POA may represent an alternative sensitive method to screen for AMI or ischemia in patients with anginal symptoms and no evidence of acute infection.