testosteroni . The 7α- and 7β-hydroxylated derivatives of [ 4- 14 C ]-EPIA produced were prepared after incubation with mycelium of Rhizopus nigricans . Each labeled steroid was purified by chromatography and identified by crystallization to constant specific activity after isotopic dilution with each authentic steroid carrier. Production yields and radio-purity measurements allowed the use of such procedures