Some patients with clinical evidence of atherosclerosis and others with diabetes were compared with appropriate controls. The intraplatelet level of platelet factor 4 (PF4) was significantly decreased in the arteriopaths and was lower in the diabetics when compared with controls. Patients with transient ischaemic attacks and stroke have even lower values. Intravenous heparin liberates large amounts of PF4 from an unknown reservoir, perhaps the endothelium, into the plasma. Arteriopaths liberated significantly more PF4 than the controls and the diabetics most. If a second heparin injection is given 24 hr after the first, the resultant plasma PF4 level was on average half that achieved after the first injection and again the patients had higher levels than the controls. Thus the reservoir originally "emptied" of PF4 by the heparin had been partially refilled in 24 hr and the reservoir in the atherosclerotic patients then contained more than the controls. Patients with atherosclerosis and especially diabetics differ from controls in the PF4 content of their platelets and in their response to heparin and in the rate of refill of the "heparin-mobilisable pool of PF4".