1. Alterations in cellular immunity have been suggested to occur in major depressed patients. 2. To investigate the populations of B-cells, T-cells and T-cell subsets in major depression, the authors utilized monoclonal antibody techniques to enumerate the number of total lymphocytes, B-cells and T-cell subpopulations in recently admitted patients with major depression or non-affective disorders. 3. The authors also studied the relationship between the immune state and hypercortisolism as measured by post-dexamethasone serum cortisol. 4. The preliminary findings from this pilot study suggest that major depressed patients may have altered cellular immunity as demonstrated by lower numbers of natural killer cells. 5. Further study will be necessary to confirm the trend for lower numbers of T-cell lymphocytes and T-cell subpopulations including helper cells, suppressor cells and natural killer cells in patients with non-suppression of serum cortisol following dexamethasone.