Data on clopheline effect on platelet aggregation, both spontaneous and ADP- or adrenaline-induced, in normal donors are reported. Clopheline is shown to slightly increase aggregation in normal subjects, exhibiting properties of a weak aggregation inductor. Where adrenaline is present in the aggregating system, clopheline undermines its platelet aggregation-inducing effect and, consequently, shows properties of a more competitive antagonist, as compared to adrenaline.