Résumé : A systematic in vitro study has been carried out on the activity of pyrazole derivatives on the aggregation of blood platelets. Tests of platelet aggregation induced by adenosine diphosphate and by Thrombofax have been used. The activity of pyrazinobutazone and of methylpyrazinobutazone is similar to that of sulfinpyrazone and of phenylbutazone, but greater than that of oxyphenbutazone; the most active pyrazole derivative is still aminopyrine.