Testosterone production by Myc. sp. NRRL B-3683 is discussed. The unex
pected finding that testosterone is not formed by single reduction of
17-keto group of 4-androstene-3,17-dione (AD) but. by a double reducti
on of both 17-keto group and doble bound of 1,4-androstadiene-3,17-dio
ne (ADD) is presented.