An aqueous extract of mycelium of Ustilago esculenta, an endophytic-like fungus, was intraperitoneally administered to mice with a model antigen (NP-BSA) and aluminum adjuvant (Alum). Blood was periodically collected from the tail veins of the mice, and the antibody titer against NP-BSA and/or BSA in the serum was measured by ELISA. The aqueous U. esculenta extract had no significant effect on the IgM and IgG antibody titers in the primary response but greatly lowered those in the secondary response. When the antibody titers of the IgG subtypes were separately measured, the ratios of IgG2b/IgG1 and IgG2c/IgG1 titers in the mice treated with the U. esculenta extract were markedly lower than those in the control mice treated with NP-BSA/Alum. The Bio-Plex^TM cytokine assay revealed that the concentrations of interferon-γ (IFN-γ) and interleukin-12 (IL-12 (p70)) in peripheral blood from U. esculenta extract-treated mice were decreased as compared with those from the control mice three weeks after the initial immunization. These results suggest that the aqueous extract of mycelium of U. esculenta suppressed antibody production in the secondary response and that changes of the Th1/Th2 balance were involved in this process.