1. This paper reports the structural analysis of proteoglycans and mucopolysaccharides extracted from a human multiple enchondroma (enchondromatosis), a benign cartilage tumor, where growth, but no calcification takes place. The tumors were located inside the phalanges of both hands of a 22-year-old patient and were obtained after surgery. 2. The proteoglycans of chondromas contain only a small amount of keratan sulfate (1.3% of total mucopolysaccharide) and the chondroitin sulfate is composed of 4- and 6-sulfated disaccharide units in approximately equivalent amounts, forming hybrid polymeric chains. Furthermore, the electrophoretic mobility of these proteoglycans in agarose-polyacrylamide large-pore gel indicates that they may occur as a single polydisperse component. This structural pattern is very similar to that of the proteoglycans present in the articular cartilage of normal human newborn and young. In contrast, the proteoglycans of adult articular cartilage contain higher amounts of keratan sulfate (25% of the total mucopolysaccharide) and very small amounts of 4-sulfated disaccharide units (7%) in the chondroitin sulfate molecules. The multiple zones observed in agarose/polyacrylamide large-pore gel electrophoresis indicate the presence of more than one polydisperse component. These findings suggest a correlation between the structural characteristics of the proteoglycans and the occurrence of growth in the cartilage tissue. 3. Although the amounts of proteoglycans extractable from chondromas and from normal young and adult articular cartilages were almost the same, the chondroma proteoglycans interacted with hyaluronic acid to a lesser extent than those from the normal cartilages. This effect may be due to structural changes in the hyaluronic acid-binding region of the proteoglycan monomers.