BALB/c mice are highly susceptible to infection with the protozoan parasite LEISHMANIA MAJOR. This susceptibility has been attributed in part to the expansion of Th2 cells and production of their cytokines, IL-4 and IL-10 and down-regulation of Th1 cytokine, INF-γ. The inability of susceptible hosts to mount the immune response necessary to activate macrophage and destroy the parasites can be due to the parasite-specific proteins that are able to modulate the immune system. In this report, we monitored the culture supernatant of the parasite in order to show the existence of immunosuppressive factor. The L. MAJOR parasites were isolated from lesion of infected BALB/c mice. Amastigotes were grown at 35 °C and promastigotes at 22-25°C in the presence of 10% FCS. Culture supernatants were harvested after 72 h of incubation and used for lymphocyte proliferation assay using lymph node and spleen cells stimulated with mitogens. We have shown that the excretory materials of promastigotes and axenic amastigotes have a potent immunosuppressive activity. Culture supernatants can suppress the proliferation of mitogen-stimulated lymph node lymphocytes (74%) or spleen cells (90%) in a dose-dependent manner. This result may show that parasite by excretory materials can influence INF-γ production by T cells and macrophage activation in order to survive within the macrophage and propagating inside the phagolysosome.