The fibrinolytic resistance of platelet-rich arterial thrombi received much attention. Clot lysis method was used to assess the in vitro fibrinolytic properties in diabetes mellitus. Platelet rich (PRP) clots were formed by addition of thrombin, and lysis was induced by tissue-plasminogen-activator. The coagulation and lysis was followed by the light scattering properties. A special pattern of good initial lysis followed by a second clotting phase was observed in more than half of insulin dependent diabetic patients, while a similar pattern of clot-lysis was only occasionally found in non-insulin dependent diabetes mellitus or in the healthy control group. Following the thrombin activation of washed, gel-filtered platelets, the supernatants possessed an inhibitory action on in vitro lysis of PPP-clots. This suppression was remarkably stronger in IDDM, along with the highest PAI-1 activity concentration ratio of the platelet lysates, compared to plasmatic levels. The relation of this special type of PRP clot-lysis resistance to diabetic vascular complications needs further clarifying and investigations.