This study was conducted to determine the effects of shipping stress on the immune system in domestic lambs (Ovis aries) and to determine the potential of the thymic peptide, thymulin, to reduce those effects of stress on the immune system. Treatments consisted of no shipping (as unstressed controls), shipping (as stressed controls) or shipping plus two doses of thymulin. The shipping procedure was conducted for two consecutive days. The responses were measured in three ways. First, the ability of peripheral blood mononuclear cells (PBMN) to respond to four different doses of the mitogen, Interleukin-2 (IL-2), was measured. Second, antibody response to a standard antigen dose over a three week period following the stress was examined. Third, the plasma cortisol concentrations in stressed versus unstressed and in thymulin treated lambs were compared. There were no differences between any of the four treatment groups (p>0.44) with respect to the animals' lymphocyte proliferative ability. Although there were no detectable differences, caution should be used in interpreting these results, because of technical difficulties encountered with a key reagent in the assay. Antibody titers were measured at weekly intervals for each of three consecutive weeks following the stressing procedure. These results also showed no treatment effect between any of the four groups (p>0.39). A comparison of cortisol levels in the four groups revealed that shipping stress increased plasma cortisol concentrations, and thymulin treatment at either dose and on both days of shipment inhibited (p<0.0001 and p<0.047, for day one and two, respectively) that stress-induced increase in cortisol. Interestingly, these results indicate that treatment with thymulin was effective in negating the stress-associated increase in plasma cortisol levels in the lambs. These in vivo data support a possible immunomodulatory function of thymulin. / Graduation date: 1997