The recent introduction of the determinations of platelet factor 4 (PF4) and beta-thromboglobulin (beta TG) by radioimmunoassay provided a new tool to obtain knowledge of in vivo platelet activation. We evaluated the plasma level of PF4 and beta TG in 14 normal subjects (mean PF4 7.7 ng/ml; beta TG 28.8 ng/ml), in 29 patients with chronic stable cardiovascular disorders (mean PF4 9.8 ng/ml; beta TG 32.6 ng/ml) and in 15 diabetics with vascular disease (mean PF4 14.5 ng/ml; beta TG 41.8 ng/ml). The great majority had normal values and no statistical differences were noted among the three groups (p greater than 0.05). Fifteen days of treatment with 150 mg daily of dipyridamole produced a significant reduction in the levels of both proteins (p less than 0.01), in contrast of the daily administration of 650 mg of aspirin, which failed to produce any significant change (p greater than 0.5). The patients and the normal subjects were also administered 3,000 USP units intravenously of porcine heparin. The values of the heparin released-platelet factor 4 (HR-PF4), evaluated 5 minutes after the injection, showed a good correlation between platelet concentration and HR-PF4 levels (z = 2.37, p less than 0.02) in the patients. The determination of standard residual following linear regression analysis of HR-PF4 indicated the presence of two distinct patient populations. One group, including the vast majority of patients, did not differ from the control (patients mean HR-PF4 111.1 ng/ml; controls: mean HF-PF4 136 ng/ml). The other group, with severe cardiovascular disease, but with normal levels of PF4 and beta TG in almost all patients and similar platelet concentrations, showed a significantly higher HR-PF4 (219 ng/ml). Neither aspirin nor dipyridamole had any effect on the level of HR-PF4. This HR-PF4 could represent a possible marker of the interaction of platelets with a seriously damaged atherosclerotic vessel wall.