1. 1. Ninety psychiatric inpatients with a DSM III diagnosis of schizophrenia, mania, or major depression were studied. 2. 2. Upon admission/transfer to the Clinical Studies Unit, and prior to discharge, measurements of symptom severity (BPRS, Ham-D, Young's Mania Scale) and blood samples were obtained. 3. 3. Erythrocytes from these paired (admission and discharge) blood samples were assayed for methionine adenosyltransferase (MAT) activity and phosphatidylcholine (PC) content. 4. 4. Comparisons were made between the changes in MAT Vmax, or % PC, and changes in symptom severity. 5. 5. For the majority of the patients (79.3% of the schizophrenics; 84.6% of the depressives; and 93.8% of the manics), clinical improvement was associated with a "normalization" of enzyme activity. The association between changes in % PC and clinical response did not achieve significant correlation. © 1988.