In vivo platelet activity was studied in 58 patients with AMI on admittance to the Coronary Care Unit, in 48 of these patients after 1 week, in 30 after 1 month and in 24 patients after 6 months. Patients were carefully selected and excluded if they had associated diseases known to increase platelet activity. In vivo activation was studied by evaluating the plasma concentration of beta-thromboglobulin (beta-TG) and platelet factor 4 (PF4) in the same blood sample. On admittance (x = 58.5) and on day 7 (x = 52.9) beta-TG values were significantly higher than those obtained in the control group (x = 29). beta-TG values were moderately elevated after 1 month (x = 37.7) and then returned to values similar to those of the control group after 6 months (x = 27.9). The simultaneous assessment of PF4 shows a beta-TG/PF4 ratio indicative for in vitro release (less than or equal to 2.5) in many patients on days 1 and 7. Moreover, the beta-TG/PF4 ratio in patients with AMI tends towards 2 when beta-TG values are high. These results may indicate a greater tendency to an in vitro platelet release in the acute phase of AMI.