The immune system has a pivotal impact on body homeostasis in mammalians, mainly through cytokine network. The cardiovascular system is one of the main targets for cytokines. For instance, during pathological immune responses such as infection, autoimmunity and tissue rejection the cytokines have many different impacts on cardiovascular system. Therefore we have designed an in vitro model of lymphocyte activation, mixed leukocyte reaction (MLR) which represents the immune response during tissue rejection or an immune dominant antigen. Human peripheral blood mononuclear cells (PBMCs) were stimulated by mytomicine-treated human leukocyte for three days and then the supernatants were harvested.Three dilutions of supernatants (1/640000, 1/320000 and 1/160000) were used to investigate the impact of cytokines in such circumstances on ISOLATED HEART. Each concentration was infused to the HEART for 10 minutes and the HEART rate and the intarventricular pressure was recorded for 40 minutes. The highest dilution, 1/640000 significantly, reduced the HEART rate in immediate and delayed manner which started during infusion and lasted for 40 minutes.However, the immediate effect has not been seen in 1/160000. Moreover, the supernatant in highest dilution, 1/640000 significantly reduced the Max dp/dt 2 and 6 minutes after infusion.In conclusion, the data shows that the human lymphocyte-activated supernatants from MLR, in very low concentrations, have negative inotropic and chronotropic effects in on ISOLATED rat HEART.