In view of the evidence that unfractionated heparins (UH) cause thrombocytopenia in vivo and platelet activation in vitro, we investigated the effect of UH administration on platelet indices likely to reflect in vivo activation. Our experiments, in healthy volunteers, show that following the intravenous administration of UH: (1) platelet aggregation in whole blood is significantly enhanced; (2) platelet counts in platelet rich plasma (PRP), prepared using citrate as anticoagulant, are significantly diminished; this effect is not observed in PRP prepared from blood anticoagulated with EDTA or in the whole blood itself and (3) the modal and median platelet volume increases significantly. We conclude that some degree of platelet activation occurs after the intravenous injection of UH.