In this study, we investigated the immunosuppressive effects of water-soluble barley ${eta}$-glucan (BP) in mouse immune cells. The BP was prepared from barley (Hordeum vulgare), and it suppressed the proliferation of splenocytes stimulated with concanavalin A (Con A), interleukin-2 (IL-2), and alloantigenic splenocytes in a dose-dependent manner (0.25, 0.5, and 1mg/mL). Furthermore, the death-inducing activities of cytotoxic T lymphocytes (CTLs) and lymphokine-activated killer cells (LAKs) in alloantigen-reactive splenocytes were significantly suppressed by more than 50%. Also, the production of IL-2 in the splenocytes decreased. These results suggest that these barley-originated polysaccharides can be used as potential immunosuppressive agents.