This study evaluates the effect of methanol extracts of three Brazilian medicinal plants on in vitro proliferation of human mononuclear cells. Lymphoproliferation assay was carried out by incubating human peripheral blood mononuclear cells from healthy donors (1 x 10(6) cells/mL) with extracts of Calophyllum brasiliense (roots), Ipomoea pes-caprae (whole plant) and Matayba elaeagnoides (bark), both at 10, 50, 100 and 200 microg/mL, alone or with phytohemagglutinin (PHA, 5 microg/mL), in 96-well microplates at 37 degrees C with 5% CO2, for 72 h. The quantification of cell proliferation assay was performed by blue tetrazolium (MTT) reduction with reading at 540 nm. Cells incubated with only the culture medium were used as negative control for cell proliferation, while the positive control consisted of cells and PHA. The results suggest that the extracts of all three studied plants induce T lymphocyte proliferation. I. pes-caprae showed immunostimulatory activity three times higher than the C. brasiliense extract, while that of the M. elaeagnoides extract was 1.5 times higher. The results demonstrate immunostimulatory effects of these three plants, therefore the continuity of these studies is recommended, in order to determine the active principles.