The effect of dietary copper (Cu) supplementation with Cu-Sulfate (CuSO₄) or Cu-Lysine (CuLy) on Cu status and bovine monocyte function was evaluated through a series of experiments. Initially, two in vitro techniques, immunomagnetic (IM) and culture flask adherence (CF), were compared for isolation of a viable, homogeneous monocyte population. The CF technique for monocyte isolation resulted in both a greater number of cells exhibiting phagocytic activity, as well as, an increased phagocytic capacity compared to monocytes recovered by the IM technique. Culture flask adherence appears to be an efficient technique for isolation of a viable, homogeneous population of bovine monocytes. Copper status and monocyte function were evaluated in beef calves supplemented with Cu over a 2 year study period. Fifty-four weaned calves were allotted to one of three Cu treatment groups in a 150 d feeding trial, Plasma Cu concentration was increased in CuLy-supplemented calves over controls and CuSO₄-supplemented calves on d 42, 84, and 126. Calves supplemented with Cu had increased ceruloplasmin activity on d 84, 126, and 150 as compared to controls. Hepatic Cu measured on d 150 was decreased in controls compared to Cu-supplemented calves. Monocyte cell number and function from CuLy-supplemented calves showed increased phagocytosis on d 84 and 126 and increased oxidative burst on d 42 and 126 compared to controls. Dietary Cu supplementation was repeated using 45 calves in a 120 d study. CuLy-supplemented heifers had increased major histocompatibility complex (MHC) class II expression on d 68, 82 and 110 compared to CuSO₄-supplemented and control group heifers. Heifers supplemented with Cu had increased plasma Cu concentrations on d 82 and 110 compared to controls. The effect of vaccination on monocyte function was evaluated in Cu-supplemented beef heifers. Vaccination with B. abortus Strain 19 increased monocyte oxidative burst, phagocytic activity, and MHC class II expression in heifers. Copper supplementation and source of Cu supplement influenced monocyte response to vaccination. Monocyte response appeared to be higher in CuLy-supplemented heifers after vaccination compared to CuSO₄-supplemented and control heifers. / Ph. D.