It is well accepted that the immune system shows circadian rhythmicity and that circadian disruption can significantly alter indices of immune function. Recently, a functional link between the endocrine and immune systems has been proposed to explain circadian rhythms in immune activity. Of particular interest is the finding that hormones such as melatonin and corticosteroid are able to exert modulating effects on lymphocyte proliferation. Previous research examining the effects of melatonin in vitro, however, has produced equivocal results. The aim of this study, therefore, was to examine the effects of melatonin and corticosteroid, both separately and together, on mitogen-stimulated human lymphocyte proliferation. Purified human lymphocytes were stimulated with concanavalin A (Con A, 4 micrograms/mL). Melatonin and/or corticosteroid were added to the culture medium during incubation. All cultures were done in quadruplicate. As expected, corticosteroid (25-1,000 ng/mL) significantly reduced proliferation by between 30 and 60% in a dose-related manner (P < 0.0001). Melatonin alone (10-1,000 fmol/mL) did not significantly affect lymphocyte proliferation. However, when lymphocytes were cultured in the presence of melatonin and corticosteroid, a significant decrease in proliferative function of 50-85% was observed (P < 0.0001). Hence, the effect of melatonin and corticosteroid combined was significantly greater than that observed with corticosteroid alone (P < 0.0001). Therefore, it appears that the in vitro effect of corticosteroid on immune function may be modulated by melatonin in physiological to pharmacological concentrations.