Dehydrogenated derivatives of corticosteroids are usually more effective than their precursors in treating diseases. In this study, the toxicity of seven water-miscible ionic liquid and three organic solvents to the biocatalyst Arthrobacter simplex UR016 was tested to evaluate the possibility of biodehydrogenation from 17α-hydroxy-16β-methyl-pregna-4,9(11)-diene-3,20-dione (HMPDD) to 17α-hydroxy-16β-methyl-pregna-1,4,9(11)-triene-3,20-dione (HMPTD) in an ionic liquid-containing system. Although most tested room temperature ionic liquids (RTILs) showed higher toxicities to A. simplex UR016 than organic solvents, bacterial growth was promoted in the presence of [EMIM](