Serum immunoglobulins were quantified in 85 consecutively admitted patients from an acute-care psychiatric in-patient unit. The patients were diagnosed as: major depressive disorder, 53; chronic schizophrenia, 12; schizophreniform psychosis, 4; substance abuse disorder, 12; and miscellaneous other diagnoses, 4. A small sub-group of these patients, which included six with major depression, one with chronic schizophrenia, and one with a personality disorder, were found to have low IgM concentrations. Immunoglobulin levels were not associated with use of medication or length of illness prior to this hospital admission. This study provides further evidence for suppression of humoral immunity in some psychiatric patients (approximately 10%).