The relative uptake of copper from ceruloplasmin and non-ceruloplasmin plasma pools, by normal and malignant cells, was investigated in vivo and in vitro, using 64Cu and 67Cu. 1. Most of the copper administered intravenously to normal and tumor-bearing rats was removed within 1 h, a substantial portion entering the liver. There were differences in the apparent avidity of individual tissue for ceruloplasmin vs VSports手机版. ionic copper, but when calculated on the basis of actual microgram absorbed, all showed a preference for ceruloplasmin. 2. Appreciable amount of copper from either source were also absorbed by the tumors, and cultured Ehrlich ascites tumor cells showed a rapid uptake and marked preference for ceruloplasmin over non-ceruloplasmin copper, as did primary rat muscle cell cultures. 3. Ceruloplasmin protein was also absorbed by normal and neoplastic rat tissues, but less rapidly than ceruloplasmin copper, as determined by administration of pure [3H]leucine- or [125I]ceruloplasmin. Copper deficiency did not accelerate this process. 4. It is concluded that, at least in rat, ceruloplasmin is the preferred plasma source of copper for normal and malignant cells, and that the copper on ceruloplasmin turns over more rapidly than the protein moiety, a finding consistent with its role as a copper transport protein. .