The effect of epiphyseal hormone melatonin on the cortisole-induced decrease in peripheral blood mononuclear cell (PBMC) proliferation and changes in T-helper type 1 cytokine (IFN-gamma) and type 2 cytokine (IL-4) expression was studied in vitro in cell culture supernatants. Cortisol (10(-6) M) decreased PBMC proliferation, suppressed IFN-gamma production, and did not influence IL-4 production. Melatonin in physiological concentrations (10(-11) M) partly reduced the cortisol-induced PBMC proliferation. This effect was not associated with changes in the IFN-gamma and IL-4 production in the cell culture.