Copper (Cu) is absorbed from the duodenum in man and chicks, from the upper jejunum in dogs, from the small intestine and colon in pigs, and from the stomach and small intestine in rats. Cu absorption is inhibited by other ions, probably at the level of the intestine. After absorption, Cu is found in the plasma bound with a protein, ceruloplasmin, in most species other than chicks and turkeys. If one assumes that phytic acid in isolated soybean protein is responsible for the majority of the Cu-binding activity, it is probable that its effect is minimal on Cu availability. Cu is complexed very readily by amino acids, peptides, and certain proteins. The ascorbic acid seemed to intensify the effect of a Cu deficiency, which was not alleviated by the addition of reserpine or estrogen. Ascorbic acid depressed the absorption of 64Cu from a ligated intestinal segment.