Organic solvents have sometimes been used to increase the solubility of water insoluble substrates for steroid transformation using immobilized whole cells, even though the cell viability is often damaged. Polymeric surfactants which form micelles in aqueous solutions could be used instead of organic solvents to solubilize the steroid. We have successfully utilized this approach by employing a poly(dimethyl siloxane)-poly(ethyleneoxide) (PDMS-PEO) block copolymer surfactant to enhance conversion of hydrocortisone to prednisolone by immobilizedArthrobacter simplex cells, without deactivation of the immobilized cells.