The fruit of the plum tree (Prunus salicina Lindl.) has been used as a traditional medicinal food in humans to enhance immunity against infectious agents and to treat cancers. However, limited information exists on the mechanisms responsible for its immune enhancing properties. In this study, the immunostimulatory effects of a methanol extract of plum fruit following methanol evaporation and dissolving in PBS were assessed by in vitro lymphocyte proliferation, tumor cell cytotoxicity, and nitric oxide (NO) production. The crude methanol extract stimulated spleen lymphocyte proliferation and NO production by cultured macrophages, and inhibited the viability of tumor cells, significantly greater than media controls. Sequential gel filtration chromatographic separation of the extract on Sephadex G-25 and Sephacryl S-200 gel filtration columns resulted in a more purified preparation that retained the ability to induce lymphoproliferation, tumor killing, and NO production. These results suggest that Prunus salicina contains immunostimulatory components that potentially may be useful in human and veterinary medicine.