Adrenosterone is reduced to 1.1-ketotestosterone by cultures of microalga Scenedesmus quadricauda. The best bioconversion yields (62%) were obtained in cultures containing 3 x 10(6) cells/mL of alga and 100 mg of adrenosterone in 400 mL of medium after 10 days at 24 degrees C with a photoperiod light - dark 16:8 h.