In a previous paper we described the synthesis and the antiproteolytic activity of N-(3- and N-(4-amidinobenzoyl)-L-amino acids. As an extension of these studies we examined the possible inhibitory effect of these compounds on thrombin, blood coagulation and platelet aggregation. Values of Ki for the binding to thrombin are independent of the nature of the amino acids side chain and superimposable with those obtained for the formation of benzamidine-thrombin adduct. These data suggest slight effect on blood coagulation. The benzamidine derivatives are more active on platelets.