The effect of the aqueous fraction of the ethanol extract of Cissampelos sympodialis Eichl. (Menispermaceae) leaves (AFL) on Concanavalin-A (Con-A)-activated spleen cell proliferation and cytokine (IL-2, IL-4, IL-10 and IFN-gamma) secretion were analyzed. BALB/c spleen cells were treated, in vitro, with different concentrations, ranging from 6.25 to 400 microg/ml of AFL either in the presence or the absence of 5 microg/ml of the mitogen Con-A. It was observed that concentrations of the AFL higher then 50 microg/ml were toxic to the cells and concentrations ranging from 6.25 to 50 microg/ml decreased the lymphocyte proliferative response in the presence of the mitogen. This inhibition of mitogen induced proliferation was not reverted by the addition of exogenous recombinant IL-2. The AFL did not significantly inhibit IL-2 secretion. In the presence of AFL there was a reduction in the levels of secreted IFN-gamma while the production of both IL-10 and IL-4 were increased. AFL did not induce the production of any of the cytokines analyzed, in the absence of Con-A. It is suggested that increased IL-10 production down regulates IFN-gamma secretion and T cell proliferative responses.