Transforming growth factor $eta$1(TGF-$eta$1) has potentials to be used as a new therapeutic agent. However, studies with TGF-$eta$ were hindered by its high cost. In this study, we developed an improved method to purify TGF-$eta$1 from human platelets, for which four purification steps were used: platelet extraction, gel filtration, cation exchange chromatography, and reverse phase high performance liquid chromatography. After a final step of purification, a pure protein with a molecular weight of 25,000 corresponding to the commercially available TGF-$eta$1 was obtained, which were confirmed by silver staining and Western blotting after SDS-polyacrylamide gel electrophoresis (SDS-PAGE). It was confirmed by the inhibitory effects of TGF-$eta$1 on a mink lung epithelial cell line that the purified TGF-$eta$1 had its biological activity, whose activity is slightly higher than that of the commercially available TGF-$eta$1. About 3.7$mug of the purified TGF-$eta$1 was obtained from 10 units of concentrated human platelets, the final yield of which is about 21%.