The objective of the study was to examine the effect of inulin level, in regard to its degree of polymerisation (DP), on microbial activity and mucosal immune system of the large intestine of pigs. A total of 56 castrated male piglets (PIC × Penarlan P76) were allocated to seven groups and fed from the 10th day of life cereal-based diets without the addition of inulin or with 1%, 2% or 3% of inulin with an average DP of 10 (IN10) or 23 (IN23). Pigs were sacrificed at the age of 50 days. Feeding IN10 diets increased fructan concentration in the large intestine compared with IN23 diets, but did not affect microbial activity, except for digesta pH and mucinase activity in the middle colon, which were greater at the 1% level compared with the control group and other IN10 diets, respectively. The concentration of secretory immunoglobulin A in the caecum and middle colon was reduced by the 1% IN10 diet compared with the control group. Pigs fed the 2% IN23 diet had a higher butyric acid concentration in the caecum and proximal colon and greater isoacid concentrations in the middle and distal colon in comparison to the control. Dietary level of IN23 did not affect secretory immunoglobulin A concentration but the count of caecal intraepithelial lymphocytes was higher in pigs on the 1% IN23 diet than on the control diet. Neither IN10 nor IN23 diets affected populations of Bifidobacterium or Lactobacillus spp. In conclusion, the effects of inulin in the large intestine of pigs depended on dietary level and DP. IN23 increased short-chain fatty acid production at the 2% level and slightly activated mucosal immune status at the 1% level.