The nature of proteins occluded in the mineralized matrix of calcified areas of atherosclerotic human aortic tissue has been investigated. In spite of their reported presence in atherosclerotic lesions, neither plasma lipoproteins, immunoglobulin G or fibrinogen could be detected among the proteins specifically trapped in the mineralized matrix and released by decalcification. However, human serum albumin was present in the decalcifying extract in a persistent complex with a more acidic protein component. Also present in the decalcifying extract was an acidic protein component with a molecular weight between 6000 and 10000 which contained both serine phosphate and gamma-carboxyglutamic acid. This component closely resembled in amino acid composition the non-albumin moiety of the serum albumin complex, suggesting that this low molecular weight, gamma-carboxyglutamic acid-containing protein component was present in the calcified matrix both in the free form and in association with human serum albumin.