Different possibilities for converting pregnenolone triacetate to prednisolone using immobilized preparations of Flavobacterium dehydrogenans, Curvularia lunata and Arthrobacter simpelex in a fixed-bed loop reactor were investigated. The effects of the carrier, substrate concentration, pH and temperature on the rate of the substrate conversion were studied also, The biotransformations were performed with a continuous or semicontinuous substrate supply. A convenient pathway for the formation of prednisolone is proposed on the basis of the results obtained.