Eosinophil cationic protein (ECP), a highly basic protein secreted from eosinophilic granulocytes, has been shown to take part in the inflammatory reaction. The involvement of ECP in fibroblast activation was therefore investigated in cell culture. Production of proteoglycans, hyaluronan and collagen in the presence of ECP was measured after incorporation of radioactive precursors and separation into different proteoglycan classes using gel and ion exchange chromatography and hydrophobic interaction chromatography. Proteoglycan accumulation in the cell layer was increased two- to fivefold at an ECP-concentration of 10 micrograms/ml. No effect on collagen, other proteins or hyaluronan was noted. Furthermore, no effect was observed on cell proliferation. The increased proteoglycan accumulation could be inhibited by addition of heparin or of antibodies to ECP. The effect could not be mimicked by the two basic peptides protamine and poly-L-lysine, speaking in favor of specificity. The increase in proteoglycan material was seen exclusively in the intracellular pool. No change of proteoglycans in the medium or the cell surface-associated pool was noted. The increase in the cell layer was accounted for by a two- to fivefold increase in free chains of heparan sulfate and dermatan sulfate. No change was seen in the proteoglycan pattern. No effect on proteoglycan synthesis or on endocytosis was noted. The increased accumulation of polysaccharide was caused by inhibited degradation of glycosaminoglycans. The half-lives of large and small heparan sulfate proteoglycans/glycosaminoglycans and dermatan sulfate proteoglycans/glycosaminoglycans in the cell layer are increased four- to sevenfold. We conclude that ECP inhibits proteoglycan degradation in fibroblasts, which indicates a role for the eosinophil in generation of fibrosis.