Lentinan (LTN) was extracted from Lentinus edodes (Berk) Sing with molecular weight of 5 x 10(5). The effects of lentinan on cellular immune function were studied in vivo by measuring the cellular delayed type hypersensitivity (DTH) to dinitrofluorobenzene (DNFB) in cyclophosphamide (Cy)-comprised mice. The effect of lentinan on T lymphocyte proliferation to Con A on splenocytes and T lymphocyte subpopulations on thymocytes and on splenocytes from normal mice were also evaluated. Moreover, the effect of LTN on production of tumor necrosis factor (TNF) from murine peritoneal macrophage was also tested. LTN was administered at doses of 1, 5 and 10 mg.kg-1.d-1. The following results were observed: LTN administration(X6) augmented the T lymphocyts proliferation to Con A in normal mice; LTN restorated the DTH to DNFB impaired by single Cy(200 mg.kg-1 and 80 mg.kg-1, ip) after using LTN for 8 or 5 d; LTN administration (X6) either decreased the percentage of L3T4+ (Th), Lyt2+ (Ts) in thymocytes or increased the percentage of L3T4+, Lyt2+ in splenocytes; LTN(X6) administration elicited release of TNF from M phi in the presence of lipoplysaccharide (LPS). These results indicate that the immunomodulating effect of LTN may be relevant to change of T cell subpopulation and increase of TNF production.