Pretreatment of male C57BL/6J Han mice with 200 mg of sulmazole (AR-L 115 BS) daily for 2 days results in an up to 4fold increase of the 7-ethoxycoumarin O-deethylase. The resulting effect was compared to that after a pretreatment with cobaltous chloride which also leads to a 4fold increase. Enzyme kinetic parameters of the 7-ethoxycoumarin O-deethylase are different for the two inducers in respect to the affinity for the substrate, but the inhibitions by metyrapone and alpha-naphthoflavone are similar. Comparison of the microsomal protein patterns after partial purification of microsomes revealed totally different patterns after sulmazole and cobaltous chloride. Sulmazole produces a 54 kDa protein band and cobaltous chloride a band at 48.5 kDa. Therefore it is concluded that in mice at least two 7-ethoxycoumarin O-deethylase activities can be induced. The electrophoretic pattern after sulmazole is different from those after 3-methylcholanthrene and isosafrole. This was also proven by enzyme kinetic investigations with ethoxyresorufin as substrate.