To evaluate the roles of transforming growth factor-β (TGF-β) on the glomerular cell proliferation and sclerosis, a new colorimetric bioassay system for urinary excretion of TGF-β has been developed by using NRK49-F cell culture. The urinary TGF-β activity was detected by this method in focal glomerular sclerosis (FGS) and purpura nephritis but not in minimal change nephrotic syndrome and IgA nephropathy. The bioassayed TGF-β activity was comfirmed by a blocking experiment using a neutralizing antibody and by Western immuno-blotting. These findings suggest that TGF-β plays an important role in the progression of the glomerular leisions, in FGS and purpura nephritis.