The metals are found attached to one, two or many ligands in biological systems and are of great importance for the life processes as metalloproteins, especially as enzymes. The trace metals are part of the metalloproteins. The metal is ingested in the food as a salt or as part of plant and animal tissues. The topic of bioavailability of metals has been reviewed by B. L. O'Dell, B. G. Shah, and N. W. Solomons, especially for humans. Actually, different species of animals may not have the same bioavailability of a metal. The lumen contains free metal, low and high molecular weight chelates of the metal, metalloproteins, and various ligands. C. H. Hill and G. Matrone fed a skim milk-glucose diet in their studies with chickens to study trace element interactions and observed antagonism between certain trace elements. The bioavailability of mineral elements is influenced by a number of factors — both intrinsic and extrinsic.