Each of 13 men was fed diets made from conventional foods for two to seven periods lasting 30 days in a metabolic ward. There was a total of 47 experimental periods. Supplementation of the basal diet with each of several bran or other fiber sources was done in no particular order; each subject was fed the unsupplemented diet for at least one period. The diet was designed to contain amounts of essential nutrients close to the Recommended Dietary Allowances; it resembled the "average" American diet as it contained 16% protein, 40% fat, and 44% carbohydrate. Only minimal variation in energy intake and physical activity of the men was permitted; body weight remained constant within +/- 2%. Copper in food, feces, and urine was measured during the last 12 days of each period. Linear regression of balance (intake minus the sum of fecal and urinary losses) revealed a daily requirement of 1.30 mg (95% confidence limits of 1.24 to 1.35 mg). On consideration of a calculated surface loss of 0.25 mg/day, the requirement is 1.55 mg/day. The requirement was unaffected by the type of fiber source. A review of published data revealed that this requirement substantially exceeds the amount of copper found in many conventional diets.