The effect of TGF-$eta$ on cell replication and collagen synthesis in primarily cultured mesangial cells from rats was studied in vitro. The TGF-$eta$ was purified from human platelets by acidic ethanol extraction and gel filtration with FPLC. The effect of TGF-$eta$ on the DNA synthesis and the collagen synthesis in mesangial cells depended on both TGF-$eta$ concentration and cell density in monolayer culture. Concentrations of TGF-$eta$ ranging from 0.1~15 ng/ml were evaluated for their effects upon DNA synthesis. In confluent cells, when compared to the growth rate in standard medium, TGF-$eta$ concentrations in the range 0.1~15 ng/ml reduced a cell proliferation up to 35%, suggesting inhibition of cell division. The effect of TGF-$eta$ on collagen synthesis in mesangial cells was biphasic. In confluent cells, the addition of TGF-$eta$ induced a 2-fold increase in the production of collagen when compared with cells grown in standard medium. The maximal effect of TGF-$eta$ occurred at 0.5 ng/ml. At higher concentrations there was a decrease in the action of TGF-$eta$ with the effects on collagen production. In subconfluent cells, however, TGF-$eta$ showed no significant effects on the DNA synthesis and the collagen synthesis at all concentrations. These results support that TGF-$eta$ may play an important role as a multifunctional regulator in the pathogenesis of glomerulosclerosis.