The mechanisms by which lactulose affects growth performance, intestinal morphology and the immune system in animals are not yet investigated in detail.Therefore, a total of 120 weanling pigs [(Landrace × Yorkshire) × Duroc, BW =(7.95 ± 1.22) kg; 21d of age] were randomly allotted to 1 of 4 dietary treatments (6 replicate pens per treatment with 5 pigs per pen) to investigate the effects of lactulose on growth performance,nutrient digestibility, blood profiles, fecal microflora, score and moisture, and noxious gas emission.All pigs were housed in an environmentally-controlled room, which provided 0.26 × 0.53 m2 for each pig.Each pen was equipped with a one-sided, stainless steel self-feeder and a nipple drinker that allowed pigs access to feed and water ad libitum.Individual pig BW and feed disappearance were recorded on d 14 and 42 to calculate ADG, ADFI, and G/F.This experiment lasted for 42 d.Dietary treatments were : NC, basal diet; PC, NC + 0.05% tiamulin) ; L1, NC + 0.1% lactulose; L2, NC + 0.2% lactulose.The results were as follows: pigs fed PC and L1 diets had greater (P < 0.05) average daily gain (ADG) and gain/feed (G/F) ratio than those fed the NC diet during d 1 to 14.The average daily feed intake (ADFI) was improved (P <0.05) by PC and L1 treatments compared with NC treatment during d 15 to 42.During the whole experimental period, pigs fed PC and L1 diets showed greater (P < 0.05) ADG and ADFI than those fed NC diet.There was no effect on the apparent total tract nutrient digestibility (ATTD) of DM and nitrogen (N) at wk 2, as well as DM and gross energy (GE) at wk 6.Pigs fed L1 diet increased (P < 0.05) the ATTD of energy compared to those fed NC diet at wk 2, and the ATTD of N in PC and L1 treatments was increased (P < 0.05) compared with NC treatment.The red blood cell (RBC) , white blood cell (WBC) and haptoglobin concentrations and the lymphocyte level were not affected by dietary treatments (P > 0.05).The concentration of fecal lactobacillus was increased (P < 0.05)by L1 treatment, whereas the concentration of E.coli was decreased (P < 0.05) by L1 treatment compared with NC treatment.Pigs fed L1 and L2 diets had lower (P < 0.05) fecal score compared with those fed NC diet.Fecal moisture was not affected by dietary treatments at both wk 2 and 6.L1 treatment decreased (P < 0.05) the fecal NH3 emission compared with NC treatment, but no effect on fecal total mercaptans, H2S, and acetic acid emission was observed (P > 0.05) among treatments.Results of the current study indicate that lactulose can improve the growth performance and nutrient digestibility, increase Lactobacillus concentration, decrease fecal E.coli shedding, as well as fecal NH3 gas emissions, and affect the fecal score in weanling pigs.So, lactulose could be an alternative to antibiotics to some extent in weanling pigs.