Effects of feeding supplements of calcium, magnesium, manganese, selenium and potassium on dietary copper bioavailability of humans were investigated. Results indicated that the calcium supplements depressed fecal copper losses and improved body copper retention as did potassium supplements. Magnesium and selenium supplementation of diets resulted in increased apparent fecal losses of copper while no effect of manganese supplementation was found. It may be that the unexpected positive effect of calcium on copper utilization was due to its neutralizing effect on the relatively high level of ascorbic acid provided by the constant background diet. Ascorbic acid is known to inhibit the absorption of copper.