Methionine adenosyltransferase (MAT) activity (Vmax) and the relative amount of phosphatidylcholine (% PC) were measured in erythrocytes of up to 30 DSM-III diagnosed manic, 17 unipolar depressed patients, and 28 normal controls. Manic subjects had significantly higher and depressed subjects significantly lower MAT Vmax than normals. The relative amount of PC was in the low range for the depressives, and in the high range for the manics. Depressive patients present, in these tests, similar abnormalities to those seen previously in schizophrenic patients. Clinical and diagnostic implications of these findings are discussed. © 1985.