Intact platelet [3H]dihydroergocryptine binding characteristics have been determined in female patients with a history of affective disorders who have been treated with prophylactic lithium. These results have been compared with results obtained from concurrent experiments in normal female controls and acutely-depressed, female drug-free patients. The platelet density of alpha 2-adrenoceptors in the lithium-treated patients is similar to that of the depressed patients as there are significantly fewer sites per platelet when compared to normal controls. It is concluded that lithium treatment with remission from affective illness is not associated with a change toward normal of alpha 2-adrenoceptor density and represents a trait in patients prone to affective disorder. The problem of comparison of results of platelet alpha 2-adrenoceptor density obtained from different laboratories is discussed with particular reference to the non-uniform choice of [3H]ligands used.