Abstract 35S proteoglycans produced by normal and neoplastic (U-937) monocytes after a 20-h pulse with [ 35S]sulfate in vitro have been isolated and compared. Both cell types produce exclusively chondroitin sulfate proteoglycan (CSPG), which are released into the medium and are not contained within the cells. The neoplastic cell-derived molecules were much larger in molecular size, due to the substitution of galactosaminoglycan chains, with an approximate M r of 60000. The corresponding chains in monocyte CSPG had an M r of approx. 20000. The latter chains were also found to be more sulfated than their neoplastic counterparts.