This project is designed to isolate human 20 alpha-hydroxysteroid dehydrogenase and purify the enzyme to homogeneity using affinity chromatography. The physico-chemical characteristics of the purified enzyme will be studied and crystallization will be attempted. The aim of the project is to extensively study the topography of the enzyme active site using affinity labeling analogues of progesterone. Human term placenta is the source of the enzyme.