Immobilization of the bacteria A. simplex by-2-13 was obtained by irradiation polymerization with an electron beam and gamma-radiation at 0°C. Prednisone acetate (P. A.) was derived from cortisone acetate (C. A.) by 1,2-dehydrogenation, catalyzed by immobilized A. simplex by-2-13. The immobilized bacteria were used 10 times in succession with a transformation percentage ranging from 77 to 86% for electron beam radiation and 79 to 99% for gamma radiation. Dehydrogenation, was carried out at pH 7.2, 34°C, 180–200 r.p.m. for 24 hours. 3% (v/v) anhydrous alcohol was added to the system at onset. Scanning electron micrographs documented the structure of the immobilized bacteria. In this work, the effect of absorbable doses of radiation and the reaction time of non-immobilized bacteria on dehydrogenase activity were studied. The effect of pH, temperature and concentration of substrate were also studied.