Experiments were conducted to study Cu toxicity in chicks fed casein or soy-containing diets containing graded levels of excess CuSO4.5H2O. Differences between diets in measures of Cu toxicity were pronounced. Addition of 800 mg of Cu/kg to a casein-dextrose semipurified diet decreased weight gains by 63%, but the same addition of Cu to a corn-soybean meal complex diet decreased gains by only 8%. At all levels of Cu supplementation (100, 200, 400 or 800 mg/kg), Cu deposition in the liver was increased markedly in chicks fed casein diets compared with those fed the complex diet. Substituting soy-protein concentrate for casein in the semipurified diet resulted in a marked decrease in Cu toxicity as measured by weight gain reductions and tissue accumulation of Cu.