Department of Clinical Endocrinology, Imperial Cancer Research Fund, Lincoln's Inn Fields, London, WC2A 3PX (Received 2 August 1974) Evidence has often been presented for the existence of multiple 3β-hydroxysteroid dehydrogenase/3-oxosteroid isomerase systems on the basis of preferential conversion of only one of the several possible substrates of these enzymes (Weliky & Engel, 1963; Neville, Webb & Symington, 1969). Three, or possibly four, substrate-specific isomerases have been demonstrated in mammalian adrenal glands (Alfsen, Baulieu & Claquin, 1965; also see Neville et al. 1969). More recently the 3β-hydroxysteroid dehydrogenase/isomerase system of sheep adrenal cortex microsomes has been isolated; purification of the enzymes by ion exchange chromatography revealed no separation of activities towards dehydroepiandrosterone (DHA) and pregnenolone (Ford & Engel, 1974). However, similar information on the human adrenal gland is lacking and since this enzyme system(s) might be important in the production of cortisol and androgens, an attempt has been made to investigate