Intravenous administration of phosphatidyl serine emulsion (30 and 60 mg/kg of body mass) was accompanied by an immediate decrease in the coagulation activity of blood, intensity and duration of which were dose-dependent. Phosphatidyl serine inhibited formation of thrombin in the prothrombin complex, activated by tissue thromboplastin or factor Xa in presence of Ca2+ in vitro. Kinetics of inhibition was similar in both cases, thus suggesting the interrelationship of phosphatidyl serine with prothrombin. Phosphatidyl serine inhibited also the effect of factor Xa on synthetic substrate BAME, interacting with the latter; these data were corroborated by study of absorbance spectra of phosphatidyl serine, BAME and their mixture. Thus, phosphatidyl serine inhibited thrombinogenesis due to a decrease of the interaction rate between factors Xa and II. Effect of phosphatidyl serine on activation of factor X required a special study. Phosphatidyl serine appears to participate in blood stabilization.