The process of digestion and absorption of human mineral nutrients, copper, zinc and iron, was analyzed from the standpoint of solubility and molecular size using homogenized composite diet samples. Part of the homogenate was digested in vitro by pepsin, followed by pancreatin, thus mimicking the human gastrointestinal process. The percentages of solubilized molecules of total copper, zinc and iron were not increased by pepsin digestion alone at pH 1.5 for 4h. After successive pancreatic digestion (20h), the copper became 63% solubilized on average, zinc about 50%, and iron about 79%. Gel chromatography of the soluble fraction after pepsinpancreatin digestion revealed that almost all of the soluble copper was localized in fractions with a molecular weight of less than 3, 000, zinc was localized in two molecular weight fractions (>30, 000 and <3, 000), and iron over a wide molecular weight range. These results suggest that low availability of iron in food might be due partly to a paucity of soluble iron complexes of molecular weight less than 5, 000.