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1. Incubation conditions were established in experiments with human-prostate homogenates for almost complete conversion of [4-14 C] testosterone into at least ten transformation products. 2. Whole homogenates of tissue with benign hypertrophy were shown to contain 3α-, 3β-and 17β-hydroxy steroid dehydrogenases, Δ 4-3-oxo steroid 5α-and 5β-reductases and unidentified hydroxylases. 3. Most of the 17β-hydroxy steroid-dehydrogenase activity was located in the mitochondria, which showed little other activity. 4. The 3α-and 3β-hydroxy steroid dehydrogenases and the 5β-reductase were located in the high-speed supernatant and required supplementation with NADPH for activity. 5. The 5α-reductase was located in both microsomal and high-speed supernatant fractions and also required supplementation with NADPH.