Abstract: Effect of a custom-made Chinese herbal formulation(CHF)and tilmicosin on the immunity of weaned piglets were investigated.CHF consisting of Astragalus membranaceus,Codonopsis pilosula,Epimedium sagittatum,massa medicata fermentata,Atractylodes macrocephala,malt,Glycyrrhiza uralensis,Angelica sinensis and Crataegus pinnatifida was crushed and passed through a 500-mesh sieve for the experiment.The weaned 3-line crossbred piglets(age 25±2days)were randomly assigned to 8groups of 30 each.CHF was added to the basal diet for Group I,II,and III at levels of 4kg·t^-1,7kg·t^-1,and 10kg·t^-1,respectively.Group IV,V,and VI were fed with CHF and 20%tilmicosin in the feed,i.e.4kg·t^-1 CHF+1kg·t^-1,7kg·t^-1 CHF+1kg·t^-1,and 10kg·t^-1 CHF+1kg·t^-1,respectively.Group VII was fed with a diet added 1kg·t^-120%tilmicosin.As control,Group VIII was fed with the basal diet alone.One day before,and 28 ds after the PRRSV vaccination,blood samples from the piglets from each group were drawn to test for PRRSV Ab level,albumin,total protein,Ig M,Ig G,and complement C3 concentration.The results showed that 28 ds after the vaccination the average serum antibody levels in piglets of Group IV,V,and VI were higher than the control,especially Group IV,were the highest among all,with significant difference(P<0.05)and low data spreading.The concentrations of total protein,albumin,IgG,and complement C3 in all groups were greater than those of control,especially Group IV(P<0.05),were the highest(P<0.05).It appeared that the administration of CHF and tilmicosin enhanced the immunity and PRRSV Ab in the piglets,the best effects conld be received in treatment with both CHF and tilimicosin.