ABSTRACT. Livestock transportation effects on the number of circulating leukocytes have been reported. However, data related specifically to the relation between acute stress levels during transport and leukocyte differentiation, including lymphocyte subsets, are lacking. This study was undertaken to evaluate the distribution of peripheral blood leukocyte differential counts, CD25+ lymphocytes and NK cells in calves subjected to truck transportation on different road types. Healthy Japanese Black calves were divided into three treatments: 1) those moved around in a mountainous area (Group M); 2) those moved around on flatland (Group F); and 3) those that were not transported (control). The plasma cortisol levels in Group M increased during transport. The increase was significantly higher at the end of transport than in the controls (P<0.05); a slight increase was noted in Group F. Total leukocytes and the neutrophil to lymphocyte ratio in Group M were elevated with neutrophilia at 2 hr post-transport (P<0.05); the former levels remained higher than those in the controls for 4 hr. The numbers of lymphocytes, monocytes, eosinophils and CD25 + lymphocytes remained unchanged throughout the observations. The number of circulating NK cells in Group M increased during transport and peaked shortly after transport (P<0.05). Subsequent to these time points, the counts in Group F showed a trend toward elevation. The circulating NK cell counts were positively correlated with the plasma cortisol level during transport (M, r=0.755, P<0.0005; F, r=0.653; P<0.005). These results suggest that circulating NK cells might be more rapidly mobilized than other leukocytes. Therefore, they might reflect acute stress levels in calves during road transportation.KEY WORDS: calves, cortisol, NK cell, stress, truck transport.J. Vet. Med. Sci. 72(6): 747-753, 2010 Livestock transportation is an acute physical stressor provoking an associated psychological response that is harmful whether viewed from either an animal welfare or economic perspective [38]. This stressor stimulates remarkable activation of the hypothalamic-pituitary-adrenal (HPA) axis, dramatically increasing the levels of glucocorticoids [9,15,40]. It has also been found that most physical stressors induce activation of the sympathetic nervous system and catecholamines that influence the immune system [14,32]. Consequently, these hormones are regarded as useful biomarkers in veterinary animals. Their stress-associated increases are generally viewed as responsible for immune depression. Furthermore, the elevation of glucocorticoids during the stress response engenders a complex of severe infectious respiratory diseases in cattle, presumably because of the altered immune function following transport [7,26].The finding that lymphocytes contain measurable concentrations of glucocorticoid and adrenergic receptors [1,29] that are down-regulated in response to stress [28] suggests that alteration of the lymphoid compartment might play a major role in the immune system's response to...