The zinc requirement and signs of zinc deficiency of the young guinea pig were investigated. One-week-old Hartley guinea pigs were fed low-zinc, semipurified diets with either 30% EDTA-treated casein or 30% EDTA-treated soybean protein as the protein source. Zinc was supplemented as ZnCO3 at 0, 3, 6, 9, 12, and 60 ppm (casein diets) and 0, 5, 10, 15, 20, and 60 ppm (soybean protein diets). Specificity of zinc deficiency was determined by rapid growth response to zinc supplementation following a 4-week period of zinc depletion. Significant growth retardation and decreased feed efficiency were seen in guinea pigs fed casein diets with 3 ppm zinc or less added, or soybean diets with 5 ppm zinc or less added. Plasma zinc concentration was significantly decreased in guinea pigs fed soybean diets supplemented with 20 ppm zinc or less. Less dramatic trends of decreased plasma zinc level were seen when low-zinc casein diets were fed. Plasma alkaline phosphatase activity showed significant decreases when 9 ppm zinc or less was added to casein diets and when 20 ppm zinc or less was added to soybean protein diets. Zinc (12 ppm) added to casein diets and 20 ppm zinc added to zinc soybean protein diets supported maximal growth rate for the 3-week duration of the feeding trials.