The invention concerns .alpha.-aminoacyl derivatives of phenyl-, phenoxy-, thiophenoxy- and phenylsulphinylalkanoic acids together with their amides, esters and pharmaceutically acceptable salts; processes for their preparation; and pharmaceutical compositions for therapeutic use in inhibiting the formation of thrombi and also in reducing the persistence of thrombi formed in the blood of warm blooded animals. Representative compounds of the invention are methyl 4-(aminoacetyl)phenoxyacetate, 4-(aminoacetyl)phenoxyacetic acid and methyl 4-(aminoacetyl)-thiophenoxyacetate, preferably as their hydrochlorides.