Comparative studies were performed on the uptake of 35SO4-labelled proteoglycans and of free 35SO4-glycosaminoglycan chains by cultured skin fibroblasts. Proteoglycans as well as protein-free polysaccharide chains are internalized by adsorptive pinocytosis. The maximum capacity for endocytosis of proteoglycans is about four times higher than for glycosaminoglycan chains when referred to disaccharide units. On a molar basis about 10 times more glycosaminoglycan chains than proteoglycans can be internalized within a given time. Since unlabelled carbohydrate chains do not compete for the uptake of labelled proteoglycans it seems likely that different receptors exist on the cell surface for endocytosis of both types of macromolecules. 14C-labelled hyaluronate was neither pinocytosed to a measurable extent by skin fibroblasts nor by intima cells cultured from the thoracic aorta. Rat liver hepatocytes efficiently internalized hyaluronate by adsorptive pinocytosis. It was concluded from competitive experiments with hyaluronate oligosaccharides that hyaluronate has to interact with at least two receptor molecules on the cell membrane to achieve endocytosis.