The authors determined ABO blood groupings for 66 manic-depressive patients diagnosed and divided into bipolar and unipolar groups according to strict symptomatic and course criteria. A significantly higher percentage of bipolar patients than unipolar patients had blood group O, while a significantly higher percentage of unipolar patients than bipolar patients had blood group A. THese findings provide support for the validity of the unipolar-bipolar distinction and are consistent with the concept that vulnerability to manic-depressive disorders may be related to membrane disturbances.