Lactic acid bacteria (LAB) have recently attracted considerable attention as treatment options for immune diseases, the incidence of which has been increasing worldwide. The ability of tumor necrosis factor-α producing LAB isolated from cheese to inhibit NF-κB activation in lipopolysaccharide (LPS)-stimulated peritoneal macrophages was investigated. Among the tested LAB, Lactobacillus casei HY7213 inhibited NF-κB activation most potently. Therefore, we measured its immunopotentiating effect in cyclophosphamide (CP)-immunosuppressed mice. When HY7213 was orally administered for 5 or 15 d, it reversed the CP immunosuppressant effect by increasing body and spleen weights, blood red and white blood cells levels, and splenocyte and bone marrow cells counts. Treatment with CP in mice markedly reduced concanavalin A (ConA)-induced T cell proliferation to 54% compared to the normal group. Oral administration of HY7213 in CP-immunosuppressed mice reversed that value to 95% of the normal group on day 15. Furthermore, oral administration of HY7213 to CP-treated mice significantly enhanced the expression of IL-2 and IFN-γ in ConA-induced splenic cytotoxic T cells, restored the CP-impaired phagocytosis of macrophage, and increased the cytotoxicity of natural killer (NK) and cytotoxic T cells derived from spleen and bone marrow against YAC-1. Based on these findings, we suggest that HY7213 may promote the recovery of immunosuppression caused by chemotherapeutic agents, such as CP, by activating NK cells, cytotoxic T cells and macrophages.