To provide lipase activity determination reagents which function by the enzymatic method. The reagents are easy to use in an ordinary clinical examination, have excellent handleability, and are excellent in accuracy and reproducibility. Lipase activity is determined with any of reagents which comprise a low-concentration buffer and a diglyceride dissolved therein. The diglyceride is used as a substrate for lipase, whereby the liquid reagents can have long-term storage stability. One of the reagents converts a monoglyceride yielded by a lipase reaction into glycerol with the aid of monoglyceride lipase, and further contains glycerol kinase, pyruvate kinase, lactate dehydrogenase, reduced NAD, ATP, and phosphoenol pyruvate. Another reagent converts a monoglyceride yielded by a lipase reaction into glycerol with the aid of monoglyceride lipase, and further contains glycerol kinase, glucose, ADP-dependent hexokinase, glucose-6-phosphate dehydrogenase, oxidized NAD or oxidized NADP, and ATP. A further reagent converts a monoglyceride yielded by a lipase reaction into glycerol with the aid of monoglyceride lipase, and further contains glycerol kinase, glycero-3-phosphate oxidase, peroxidase, and a dye which colors in the presence of hydrogen peroxide.