(Rece ived 22.1.1985; Accepted in rev ised fo rm 22.2.1985

i^le have invesEigated the anti thronbin II I independent effect of crude heparin, tr^ro heparin fract ions and a heparinoid on in vitro thronbin-induced platelet act ivat ion. Thrornbin-induced platelet factor Va generation and thrornbin plus col lagen-induced platelet prothrombin convert ing activi ty were tested. Crude heparin was a more potent inhibitor of these reactions than the fract ions or the heparinoid. The inhibitory action of the heparins was found to be the result of a direct effect on thrombin and not of an effect either on platelet act ivat ion functions or on the assembly or functioning of the proEhrombinase complex. Probably this heparin inhibit ion is due to the masking of secondary macromolecular sub-strate binding sites on the thrombin molecule. We found no corre-lat ion between IC.n values and the anti thrornbin II I-dependent an t i th rombin spec id i "c ac t i v i t ies o f the hepar ins. Th is suppor ts the not ion tha t hepar in p roper t ies o ther than the i r a f f in i t y fo r an t i-thrombin II I may contr ibute to the action of this drug in blood coagu la t ion