We studied the effects of fluoxetine, a non-tricyclic antidepressant drug that selectively inhibits re-uptake of serotonin by presinaptic neurons in the brain, on cellular immune responses in mice exposed to a chronic auditory stressor. The natural killer (NK) cell activity was reduced after 4, 8, 12, 16 and 20 days of stress exposure with a partial recovery on days 16 and 20. Daily treatment with fluoxetine partially reversed these adverse effects of stress in a dose-dependent manner. Significant differences appeared when fluoxetine was administered at 2 mg/kg and maximum effect was reached at doses of 5 mg/kg. The capacity of T cells to generate cytotoxic T-lymphocytes (CTL) in mixed lymphocyte cultures and in vivo was reduced after 4 days of stress application and this effect was partially reduced when mice were injected with 5 mg/kg of fluoxetine. Nevertheless, in our experiments, fluoxetine did not significantly affect the cellular immunity in unstressed mice. In conclusion, fluoxetine seems to partially recover the adverse effects of chronic stress on cellular immune response.