Blood was obtained from 63 consecutive patients within 24 h period after the admission to a coronary care unit for the determination of plasma platelet factor 4 (PF-4) concentration. 28 of the subjects proved to have an acute myocardial infarction (MI), 24 had evidence of ischaemic heart disease (IHD) but no MI, and the remaining 11 patients had no signs of IHD. 40 healthy subjects served as controls. The mean PF-4 value in the MI group was 10.5 +/- 0.8 ng/ml. The corresponding values for patients with and without IHD were 8.7 +/- 0.6 and 8.3 +/- 0.6 ng/ml, respectively. The control mean (5.4 +/- 0.3 ng/ml) was significantly lower (P less than 0.001) than the means for all 3 groups of patients studied. The difference between the group of MI patients and patients with IHD as well as patients without IHD was only of borderline significance (0.10 greater than P greater than 0.05).