We studied platelet 3H-imipramine binding and family history of affective illness in 26 patients with bipolar affective disorder and 24 patients with unipolar affective disorder. The density of platelet 3H-imipramine binding sites (Bmax) was significantly lower in bipolar patients with family history of affective illness than in healthy controls; however, there was a considerable overlap in Bmax values between the familial cases and the normal controls. A similar trend was observed in familial cases of unipolar disorder but the differences fell short of statistical significance. The nonfamilial cases of affective disorder did not differ from the healthy controls. The possible role of reduced platelet 3H-imipramine binding as a genetic vulnerability ‘marker’ in affective disorder was discus