Various beneficial effects have been described for fermented papaya preparation (FPP: SAIDOPS501) based on its antioxidative and antiinflammatory functions. The present study was designed to determine the effects of FPP on carcinogenesis in vivo, and immunomodulatory function in vitro. Mice were injected with RL male 1 cells subcutaneously or 3methylcholantherene (MCA) intravenously to induce cancer and orally or intraperitoneally treated with FPP solution. Human peripheral blood mononuclear cells (PBMC) were obtained from healthy volunteers and patients with atopic dermatitis, treated with FPP, and subjected to measurement of cytokine production and changes in Foxp3expressing regulatory T cell (Treg) stimulated with phytohemagglutinin (PHA). Administration of FPP suppressed tumor size and the incidence of malignancy. In vitro, treatment of PBMC with FPP induced IL1?, TNFα and IFNγ production. Moreover, FPP suppressed proliferation of PHAstimulated Foxp3expressing Treg. These results suggest that FPP has chemotherapeutic properties.