The protein-bound polysaccharide extracted from a fungus, PSK, has been used as a biological response modifier in the treatment of cancer patients in Japan for over 16 years. The administration of PSK to tumor-bearing rodents inhibited tumor growth and modulated immune responses. Recently, an in vitro study has revealed that PSK is a strong inducer of cytokine gene expression and production in human peripheral blood mononuclear cells (PBMC). To establish whether PSK has cytokine-inducing activities in vivo, we have orally administered PSK (1 g, the clinical dose) to 12 healthy volunteers and 9 gastric cancer patients who had undergone gastrectomy, and assessed the gene expression for cytokines in PBMC of each subject. As determined by the reverse-transcribed polymerase chain reaction method, the induction of gene expression for both tumor necrosis factor alpha and interleukin-8 (IL-8) was detected in PBMC from 5 of the 12 healthy volunteers (42%) and 4 of the 9 patients (44%). Furthermore, the concentration of serum IL-8 was elevated in 5 healthy volunteers given PSK orally, who had shown induction of IL-8 gene expression, as detected by enzyme-linked immunosorbent assay. These findings indicate that responsiveness of PBMC to PSK, in terms of gene expression and production of cytokines, varies among individuals. Thus, when using PSK to treat cancer patients, it seems advisable to select patients on the basis of their responsiveness to PSK. We speculate that the cytokines induced by PSK might mediate the immunoenhancing action of this agent in vivo.