In the previous report, we described the in vivo antitumor activity of PJ-4, a protein-polysaccharide fraction prepared from the culture filtrate of an insect-born fungus, Paecilomyces tenuipes DGUM 32001. In the present study, we elucidated the immunomodulating activity of PJ-4 on the BALB/c mouse splenic lymphocytes using flow cytometrical techniques. As a result, PJ-4 was found to stimulate the lymphocytes not only to form lymphoblasts but also to express CD25 (IL-2 receptor $alpha$ chain) molecule, which is well known as a T cell activation marker. More interestingly, its T cell stimulatory activity was more strongly exerted on CD8$^{+}$ T cells than on CD4$^{+}$ T cells. All these data suggest that PJ-4 exerts its antitumor activity at least partly through stimulation of T cells which play major roles in the cell-mediated immune system.tem.