We hypothesized that oxidative processes in myocardial tissues initiated by ischemia also cause the same kind of changes in blood proteins and lead to oxidation of sulfhydryl groups (SH). To test this hypothesis, the ratio (%MA) of mercaptalbumin to total albumin (= mercaptalbumin + nonmercaptalbumin) and the total SH contents of serum were measured in patients with coronary artery disease. There was a positive correlation between %MA and total SH contents and both values were reduced in proportion to the severity of the coronary artery disease. Furthermore, these values were lower early hours after the onset of acute myocardial infraction than 4 weeks later. These results support our hypothesis, and may provide a basis for the administration of agents which can counteract oxidative stress in order to protect myocardium in patients with coronary artery disease.