Selenium deficiency in dairy cows reduced the ability of blood and milk neutrophils to kill bacteria. Chemotactic migration of neutrophils was reduced by Se deficiency in goats and in vitro addition of Se to bovine neutrophils and macrophages has enhanced their migration. Bovine mammary endothelial cells grown in Sedeficient cell culture media were found to exhibit enhanced neutrophil adherence when stimulated with tumor necrosis factor (TNF)-a, IL-1, or hydrogen peroxide. They suggested that Se deffciency may cause tight adhesion between neutrophils and endothelial cells, and hinder neutrophil migration to the infection site. In dairy cows, whole blood Se concentrations were positively corre- lated with neutrophil adhesion. Dietary Se can also affect cell-mediated immunity in dairy cows. Peripheral blood lymphocytes isolated from Se-deficient cows exhibited a reduced response to mitogen stimulation with Con A. Impaired response of lymphocytes from Se-deficient cows may be related to altered arachidonic acid oxidation by lymphocytes via the 5-lipooxygenase pathway. Lymphocytes from deficient cows produced less products of arachidonic acid oxidation when stimulated, specifically 5-hydroxyeicosatetraenic and leukotriene B4.