Forty-eight male lambs were used to evaluate the effect of dietary supplementation of mannanoligosaccharide (MOS) with or without parenteral Escherichia coli injection on their growth performance, feed conversion efficiency, blood metabolites, total serum immunoglobulin G (IgG) levels and antibody response. Lambs were randomly assigned to four groups of 12 animals each. In groups C (control) and CE (E. coli challenged), animals were fed commercial concentrate pellets and hay (50:50), and in groups M (MOS) and ME (MOS + E. coli challenged), animals were fed commercial concentrate pellets including MOS at 0.2% and hay (50:50). At day 15 and 30, animals in groups CE and ME were injected subcutaneously with 1 ml of phosphate buffered saline (PBS) suspension containing 10(6) cfu of heat inactivated non-toxigenic E. coli O157:H7, while animals in C and M groups were injected subcutaneously with 1 ml of PBS. The experimental period was 45 days. Data indicated that body weight of lambs at the end of the study were statistically non-significant among the groups. Blood metabolites, i.e. total protein, albumin, calcium and phosphorus concentrations were not affected significantly by MOS supplementation. However, administration E. coli lowered (p < 0.05) total protein, albumin and calcium concentrations in the serum on day 30. The IgG level was not different between groups. However, on day 45, the total IgG level was found to be higher (p < 0.05) in lambs that had received MOS and E. coli than in other groups. Application of MOS did not have any effect on the antibody response to E. coli as OD values.