Therapeutic concentrations, added in vitro, of a low molecular weight heparin (LMWH), nadroparin (Fraxiparine), inhibit thrombin-induced platelet shape change (PSC), an early stage of human platelet activation. PSC was monitored by measuring the median platelet volume (MPV) using a high resolution channelyzer. The inhibitory action of nadroparin was specific to thrombin since this LMWH did not influence PSC induced by other agonists. In whole blood, unfractionated heparin (UH) but not nadroparin, significantly enhanced spontaneous platelet aggregation and increased MPV when compared with LMWH or saline controls. In conclusion, the LMWH, nadroparin, exerts less platelet activation than UH.