Microbial transformation of chlormadinone acetate to delmadinone acetate was investigated using free and immobilized cells of Arthrobacter simplex ATCC 6946 and Bacillus sphaericus ATCC 13805 in liquid-liquid biphasic system and a liposomal medium. For liquid-liquid biphasic system, n-decane, n-octanol, chloroform and butyl acetate were used as organic solvents. In liposomal medium, chlormadinone acetate was entrapped in liposomes composed of phosphatidylcholine and cholesterol. For both immobilized and free suspension biocatalysts, yields were higher in the aqueous medium compared to in liquid-liquid biphasic media. In all cases, the highest yields were obtained in the aqueous system. For the aqueous medium, a product yield of similar to48% was attained at 3 h using the free suspension A. simplex cells. The aqueous medium afforded similar to47% product yield at 48 h when freely suspended B. sphaericus was used. (C) 2003 Elsevier Inc. All rights reserved.