Epinephrine‐induced platelet aggregation. A twin study**

A study in healthy adult male twin pairs (17 MZ, 15 DZ) was devised to examine the genetic influence on epinephrine-induced platelet aggregation. Intraclass correlations of for MZ, 0.43 for DZ, and 0.61 for repetitive experiments in the same subjects point to the influence of genetic factors. The antagonizing effect of phentolamine on platelet aggregation did not prove to be under genetic control. There was no significant correlation between epinephrine concentrations which induce half-maximal aggregation and number of alpha-adrenergic receptors as measured by 3H-yohimbine binding. In one MZ twin pair concordantly, and also in two unrelated persons, epinephrine was unable to induce complete platelet aggregation. This phenomenon is not related to the number of alpha-adrenergic binding sites on platelets.