Human antithrombin III (AT III) products have been developed by two companies, Green Cross Co.Ltd.and Hoechst Japan in Japan.These two companies have recently modified the purification methods for AT III.New products from Green Cross and Hoechst Japan had 1.7-f old and 1.9-fold higher activities than the old type products, respectively.A single band was observed by SDS-polyacrylamide gel electrophoresis, and its molecular weight was evaluated to be about 63, 000 daltons.Two bands were detected by isoelectric focusing, and the isoelectric point of the major band was about 5.10.On the other hand, two peaks were observed in the old AT III product from Green Cross by use of high-performance liquid chromatography.The molecular weight of the major peak was approximately 60, 000 daltons, while that of the minor peak was about 159, 000 daltons.The precipitation of the AT III products indicated completeidentity by the double immunodiffusion assay.From these results, the quality of the new AT III products from both manufacturers were confirmed to be improved.