Transforming growth factor-β regulates production of proteoglycans by mesangial cells. Accumulation of glomerular extracellular matrix is a prominent feature of most forms of progressive glomerular disease. Since some growth factors may play a role in extracellular matrix production, we examined the effects of transforming growth factor-β (TGF-β), interleukin 1, platelet derived growth factor, and tumor necrosis factor on the production of extracellular matrix components by cultured rat mesangial cells. In control experiments we found that mesangial cells produced two distinct proteoglycans identified as the small chondroitin/dermatan sulfate proteoglycans biglycan (PG I) and decorin (PG II) by showing that their mobility on SDS-PAGE changed upon digestion by chondroitinase ABC, and that they reacted with antibodies raised against synthetic peptides from the core protein sequence of human biglycan and decorin. Exposure to TGF-β for 48 hours stimulated an 8- to 10-fold increase in the biglycan and decorin bands, and induced a structural change detected as a shift in electrophoretic mobility. TGF-β did not demonstrably affect the production of other matrix proteins by the mesangial cells. The other growth factors tested had no comparable effect on the production of proteoglycans or other extracellular matrix components by these cells. Our results show that TGF-β is unique among growth factors in its regulatory effects on mesangial cell proteoglycan production. The release or activation of TGF-β during glomerular injury could mediate the accumulation of proteoglycans in the extracellular matrix and predispose the kidney to development of glomerulosclerosis.