The aim of this study was to identify theantimicrobial substances in the sacculus rotundus and evaluate their effects on mammalian immune function and molecular mechanisms.Chromatographic techniques were used to purify antimicrobial substances.Components with antimicrobial activity were analyzed by NanoLC-ESI-MS/MS.Immunosuppressed mice were given crude protein extract or purified antimicrobial protein by gavage for 4 weeks,and serum cytokine levels were measured.Splenic lymphocyte proliferation assays were also performed.We obtained histone cluster 1,H2bc-like,which had antimicrobial activity.We termed this protein rabbit sacculus rotundus antimicrobial histone(RSRAH),according to its primary source.Sequence analysis and alignment suggested that EKTPVKKKARKSAGAAKRKA was a core segment of the protein mediating its antimicrobial activity.Treatment with high doses of the crude protein extract for 14,21,and 28 days significantly increased interferon-γ(IFN-γ)levels compared to the contro.l Interleukin(IL)-4 levels were also increased significantly on days 7,14,21,and 28 in the experimental group.Moreover,treatment with the crude protein extract stimulated a significant increase in lymphocyte proliferation on days 14 and 28.Administration of antimicrobial protein isolated from the rabbit sacculus rotundus to immunosuppressed mice increased serum IFN-γand IL-4 levels and induced lymphocyte proliferation,thereby promoting cellular immune function in immunosuppressed mice.