This study addressed the effects of a commonplace stressful event on interferon production and natural killer (NK) cell activity and numbers. The quantity of interferons (IFN) produced by concanavalin A stimulated leukocytes obtained from 40 medical students during examinations was significantly lower when compared with IFN levels produced by peripheral blood leukocytes (PBLs) taken 6 weeks earlier (baseline). In addition, three different assays measuring NK cells also showed significant decrements during examinations when compared with baseline samples. These assays included (a) lysis of MOLT-4 target cells, (b) percentage of anti-Leu-7 + (NK) cells, and (c) percentage of large granular lymphocytes. Self-report data documented the significantly greater distress associated with examinations in comparison with baseline samples. The data have implications for immunosuppressive disorders and stress-associated illnesses.