Platelet 3H-imipramine binding was investigated in 31 control subjects and 19 hospitalized bipolar patients, either in the hypomanic or the depressed phase of illness. The mean Bmax value in the bipolar depressed patients did not differ significantly from that in the control subjects or the hypomanic patients. Differences in timing of the assay after blood collection, membrane preparation, protein content used in the assay, or binding of radioactive ligand to the equipment do not appear to explain the discrepancy between these results and previous findings.