Dendritic cells (DC) are the most potent antigen-presenting cells that induce specific anti-tumor immunity. To obtain potent efficacy of immunotherapy using infusion of activated DC, it is necessary to overcome defective function of DC in tumor-bearing patients. We examined whether the treatment with PSK, a biological response modifier derived from Basidiomycetes, could allow DC to avoid inhibition of functional maturation by tumor-derived factors in vitro. CD14+ monocyte-derived DC were generated by stimulating with granulocyte-macrophage colony-stimulating factor and interleukin (IL)-4 in the presence or absence of PSK (100 microg/ml), by exposure to a tumor culture supernatant (TSN) of MKN-45P human gastric cancer cells. TSN-exposed DC were not effective in inducing cytotoxic T lymphocyte-mediated growth inhibition of target HT29 human colon cancer cells. In contrast, the presence of PSK significantly resuscitated the defective cytotoxicity. This beneficial outcome was accompanied by an increase in phagocytic activity as measured by fluorescein isothiocyanate-conjugated dextran, expression of CD83 (maturation-specific phenotype), overexpression of a CD86 co-stimulatory molecule, preserved production of IL-12 that plays a key role in the induction of Th1-type immune regulations, and protection against TSN-induced apoptosis of DC. These results demonstrated that PSK overcomes defective maturation of DC exposed to tumor-derived factors in vitro, and suggest the efficacy of PSK in DC-based immunotherapy in cancer patients.