Provided is a method for assaying arylsulfatase activity in aqueous system, which comprises a step for reacting, in an aqueous reaction system with a high ionic strength, arylsulfatase with a substrate thereof, said substrate releasing a fluorophore or a chromophore when subjected to the action of arylsulfatase. Also provided are: a lactase preparation having a lactase activity of 4,000 NLU/g or greater, said lactase activity being assayed by the FCC IV method, and an arylsulfatase activity not greater than 0.1%, said arylsulfatase activity being assayed by the fluorescence method according to the present invention for assaying arylsulfatase activity in aqueous system on the basis of the lactase activity a method for producing said preparation and a dairy product with the use of said preparation.
