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As far as the authors' knew, there was few report on the effects of composite antibacterial peptide on humoral immunity in weaned piglets.The aim of this study was to investigate the effects of dietary antibacterial peptide (ABP) supplementation on the serum IgG, IgM, IgA, CSF-Ab, and CH50 levels in piglets.Ninety weaned piglets (Landrace x Yorkshire x Duroc;(21 ± 2) d of age;average weight of (8.24 ± 0.67) kg) were randomly allotted to five groups, including the control group, the antibacterial peptide groups containing 250,500 and 1 000 mg/kg antibacterial peptide, respectively, and the astragalus polysaccharide group with 400 mg/kg astragalus polysaccharide.All Piglets were vaccinated with CSF vaccine at 21days of age.After 5 days adaptation to dietary daidzein,the whole experimental period was 28 days.Blood samples were collected at 32,39,46, and 53 days of age.The results showed that ABP as dietary additive increased the levels of CSF-Ab, CH50, immunoglobulins, and the effects of ABP supplementation were dose-dependent.Piglets fed with 250 mg/kg ABP had significantly higher (P <0.01 or 0.05) the level of IgM at 39,53 days of age, and the value of CH5o among the whole experimental period.Supplementation with 500 mg/kg markedly increased (P < 0.01 or 0.05) the value of CSF-Ab on 39,46 days, the levels of IgG, IgA on 39,46,53 days, and the values of IgM, CHs0 among the whole experimental period.High dosage (1 000 mg/kg) of ABP supplementation significantly improved (P < 0.01 or 0.05) the levels of IgG,IgA at 39,46,53 days of age, and the values of CSF-Ab, IgM, CH50 among the whole experimental period.Supplementing weaned piglets diets with ABP enhanced the humoral immune responses of weaned piglets by improving the levels of CSF-Ab, CH50, and immunoglobulins.The optimum add dosage of ABP was found to be 1 000 mg/kg for 4-week adding.