The objective of the study was to determine the effect of experimental Mannheimia haemolytica infection in sheep fed colostrum and dam or cow milk after birth on serum IgG levels. Ten Chios newborn lambs were fed ad libitum colostrum and dam milk at their dam's side (control group). Other 10 lambs were separated from their dams immediately. These lambs were not allowed to suckle normally but were fed cow milk with feeding bottle (experimental group). When the sheep were one-year-old, the study was continued. The animals were inoculated intratracheally with 1 x 10(9) log-phase Al. haemolytica organisms. Blood samples were collected from the jugular vein once before bacterial inoculation and on days 1, 4, 7, 10, 13, 16, 19 and 22 after the inoculation. Antibiotic treatment was applied on day 22. Blood samples were also collected on days 1, 4 and 7 after the treatment. IgG levels were significantly higher in the control group than the experimental one on days 0, 1, 4, 7, 10, 13, 16, 19 and on day I after treatment. The levels in the control group gradually decreased. In the light of the obtained findings, it may be suggested that the lambs non-sucking their dams might suffer, as one-year-old sheep, from an insufficiency of the immune system, and that the sheep compensated this disorder with own immune systems.