Eleven new iminosugar derivatives were synthesized and the effects of these synthetic iminosugars on proliferation of the mouse splenocytes were evaluated by the cell counting kit-8 (CCK-8) assay. The preliminary structure–activity relationships were deduced from the experimental data. It was found that iminosugar 14 (IC₅₀ = 22 μM) and its C-5 epimer 15 (IC₅₀ = 45 μM) displayed the strongest inhibitory effects, and might hold potential as immunosuppressive agents.