In order to establish the effect of an increased dietary selenium supply on platelet glutathione peroxidase (EC 1.11.1.9.; GSH-Px) and platelet aggregation, six healthy subjects were supplemented with 200-mu-g Se as Se-rich bread for 6 weeks. Another six subjects received low-Se bread and served as controls. Platelet GSH-Px activity increased significantly in the supplementation group, whereas no effect could be observed on platelet aggregation. The results of this study do not support the hypothesis that an increased dietary Se intake has a favourable influence on platelet aggregation.