BACKGROUND: Arthrobacter simplex cells immobilised in sodium cellulose sulfate/poly-dimethyl-diallyl-ammonium chloride microcapsules were used for the microbial dehydrogenation of 11 alpha-hydroxy-16 alpha,17-epoxyprogesterone to 11 alpha-hydroxy-16 alpha,17 alpha-epoxypregn-1,4-diene-3,20-dione in an aqueous/organic solvent two-liquid-phase system, which is a key reaction in the production of glucocorticoid pharmaceuticals. The aim of the study was to establish a suitable aqueous/organic solvent two-liquid-phase system for performing semi-continuous production in an airlift loop reactor by encapsulated A. simplex cells with the addition of suitable surfactants to achieve a higher yield of the product. RESULTS: n-Hexane was selected as the most suitable organic solvent. In optimised Tween-80 emulsion feed mode the conversion in the airlift loop reactor was as high as 97.54% when the time of reaction was 2 h, and the reaction time was greatly shortened. In semi-continuous production the cultivation with immobilised cells was carried out for five batches in total. The conversion in each batch was above 95% and the enzymatic activity still remained quite high after five batches of biotransformation. CONCLUSION: The results showed that performing the conversion by this method shortened the reaction time and increased the productivity, thus demonstrating the great potential of the method for the dehydrogenation of 11 alpha-hydroxy-16 alpha,17-epoxyprogesterone. (C) 2008 Society of Chemical Industry