The mycelial mass of the fungus Polyporus Squamosus strain 64 (PS-64) was disintegrated by mechanical and ultrasound treatments. After centrifugation, the supernatant containing the disintegrate was dialyzed and lyophilized. The resultant PS-64 extract was subsequently investigated as an immunomodulator of IgE and IgG responses to ovalbumin (OA) in (CBAxC57BL/6)F1 mice using passive cutaneous anaphylaxis (PCA) and enzyme-linked immunosorbent assay (ELISA), respectively. Multiple injections of PS-64 extract in doses of 1.5, 15, and 150 mg/kg administered before the primary or secondary immunization of mice with OA resulted in a dose-dependent inhibition of both IgE and IgG antibody responses to OA. In contrast to the inhibition of the anti-OA IgE response noted during the entire 3-week observation period, the anti-OA IgG response was restored to control level by the third week of secondary immunization. The glass microfiber-based whole blood histamine release assay demonstrated that various concentrations of the PS-64 extract did not influence histamine release induced either by anti-IgE or by specific allergens from basophils derived from whole blood of allergen-sensitized patients. Using the hemolytic plaque assay, significant suppression of IgM-secreting cell formation was noted in (CBAxC57BL/6)F1 mice administered various doses of the PS-64 extract before immunization. The PS-64 extract inhibited the in vitro proliferation of human mononuclear cells upon stimulation with phytohemagglutinin (PHA). In a dose-dependent manner, the PS-64 extract also inhibited delayed-type hypersensitivity reaction and skin graft rejection, similar to the effect noted with usage of Cyclosporin A (CsA) in (CBAxC57BL/6)F1 mice. Our investigation suggests that the immunomodulatory effects of PS-64 should be studied further for potential clinical therapeutic utility.