Young broiler chicks were more sensitive to copper toxicity when they were fed diets deficient or marginal in calcium content. Growth rate was depressed and liver copper concentration was increased under these conditions. Chicks fed a casein-gelatin diet were more sensitive to copper toxicity than those fed a corn-soybean meal diet. Addition of phytic acid to the casein-gelatin basal diet enhanced copper toxicity as evidenced by effects on growth rate and liver copper content. Measurements of intestinal and biliary copper content suggested that the influence of calcium on copper toxicity was mediated via intestinal absorption rather than through influences on copper excretion.