The effect of the new synthetic compound 2-[p-(1-oxo--2-isoindolinyl)phenyl] butyric acid (K 3920) on platelet aggregation was assessed in vitro on human platelets. ADP, epinephrine and collagen at different concentrations were employed as inducing agents. From the results of the present study it appears that K 3920 inhibits human platelet aggregation in vitro, probably by acting on the release reaction.