The in vitro effect of the garlic derivative alliin, on the mitogen induced peripheral blood mononuclear cell (PBMC) proliferation and cytokine production was examined. In addition, its effect on the engulfing capacity of phagocyting cells was evaluated. The results showed an increase in pokeweed mitogen (PWM) induced cell proliferation, IL-1beta and TNF-alpha production, as well as in the engulfing capacity of both percentage of phagocyting cells and number of latex particles phagocytized by each individual cell. The Con-A induced cell proliferation and IL-6 production decreased following incubation with alliin, whereas PHA-induced cell proliferation, IL-2 and superoxide anion generation remained unchanged. It is concluded that alliin in vitro exerts an immunomodulatory effect on certain functions of the peripheral blood cells.