Procollagen-induced platelet aggregation, platelet retention (Hellem II), and platelet factor 3 availability (PF-3) were determinded in a group of healthy volunteers,acetylsalicylic acid (ASA) treated patients and in uraemic patients. ASA ingestion did not influence platelet retention and PF-3 availability. Uraemic patients had apronounced decrease in platelet aggregation and retention and PF-3 availability revealed inconsistent alterations. The relevance of the tests and the benefit of an ASA treatment for prophylaxis of thrombosis in Climino fistulae are discussed.