Using the alpha 2-adrenoceptor selective radioligand [3H]yohimbine maximum binding capacities (Bmax), dissociation constants (KD), and the affinity (KI) of adrenaline for the binding sites were measured in platelets obtained from a group of 42 normal individuals. These results were compared with the efficacy of adrenaline induced aggregation in platelet rich plasma in the same subjects. Wide interindividual variation was found in Bmax values as measured in whole platelets (sites/platelet) or platelet lysates (fmol/mg protein) and in KI values for adrenaline displacement of [3H]yohimbine binding. Adrenaline's ability to induce platelet aggregation measured as the concentration required to achieve half maximal aggregation assessed from dose-response curves showed similar interindividual variation. No correlation was found, however, between alpha 2-adrenoceptor status as assessed by radioligand binding studies and the functional studies. These results suggest that the present understanding of the relationship between alpha 2-adrenoceptor occupation and platelet aggregation may be incorrect.