The Delta(1)-dehydrogenase activity of Pseudomonas fluorescens was noticeably enhanced (1.3-fold) in the presence of ethylenediaminetetraacetic acid (EDTA), and the prednisolone yield was increased 1.2-fold in the presence of H2O2 (0.6 ml/l). Macroelement (K+, Na+, Ca2+ and Mg2+) stimulated the production of prednisolone at different concentrations. The production of prednisolone at 64.7% yield was achieved when the culture medium was supplemented with 0.6 g/l NaCl at the time of substrate addition. The addition of MnSO4.H2O and FeCl3.6H(2)O (0.06 g/l) increased the capacity of the tested organism to transform cortisol (1.3- and 1.2-fold, respectively). No significant increase of prednisolone yield was shown using 1 g/l sodium citrate or sodium benzoate, while sodium acetate or sodium oxalate in the same concentration inhibited the Delta(+)-dehydrogenation of cortisol. Optimum substrate concentration was 5 mg/50 ml, while increasing the concentration of cortisol substrate to 50 mg/50 ml reduced the prednisolone yield. (C) 2002 Elsevier Science Ltd. All rights reserved.