Many studies have shown that melatonin plays a fundamental role in neuroimmunomodulation. There are known differences between mammals and birds in immunomodulatory function of melatonin exerted in vivo. In present study the effect of exogenous melatonin on chicken lymphoid cell proliferation in vitro was examined. Melatonin alone (10(-10)M-10(-5)M) did not exert any effect on the proliferation of the chicken thymocytes, splenocytes and lymphocytes from the bursa of Fabricius. On the other hand, melatonin addition strongly inhibited the proliferation of PHA-activated thymocytes and splenocytes from young chickens. The effect of melatonin was hormone- and mitogen-(PHA) dose-dependent; the most pronounced effect was obtained at low PHA and melatonin concentrations i.e., 1.6-3.125 microg/100 microl of PHA and 10(-10) M-10(-7) M melatonin, respectively. Prior immunization of chickens with sheep red blood cells (SRBC) inhibited the proliferative response of splenocytes to the same dose of PHA and, consequently, the effect of melatonin addition was not observed. The results reported demonstrate a direct inhibitory effect of melatonin on in vitro activated chicken lymphocytes.