The influence of a 4-weeks therapy with 500 mg ticlopidine daily on platelet function parameters was examined in 10 male healthy volunteers aged 20–33 years in order to extend the knowledge on the antiplatelet activity of this substance. Ticlopidine significantly (p < 0.01) affected ex-vivo platelet aggregation induced by ADP and increased platelet sensitivity to the antiaggregatory action of PGI2. Generation of TXB2 from endogenous substrate during spontaneous clotting of blood (serum-TXB2), conversion of exogenous radio-labelled arachidonic acid into TXB2 and MDA-formation in isolated platelets were unaffected by the treatment. The TXB2-level in plasma of volunteers, however, was decreased, after administration of the drug. The diminished α-granule content liberation (β-thromboglobulin: p < 0,01; PDGF: p < 0,01; PF4 not significant) indicates that ticlopidine induces a decrease in platelet activity. The beneficial effect on release reaction is not associated with a decrease in TXA2-formation. Our results demonstrate that ticlopidine inhibits platelet activity, especially the PDGF-release. These results confirm the value of this drug in the prevention of atherosclerosis and its thromboembolic complications.