An assay method for amylase activity in a biological specimen such as serum, saliva or urine. The enzyme amylase in the specimen is used to decompose a substrate which is a glucose polymer having a modified reducing terminal glucose residue or a cyclic glucose polymer. A component of the decomposed substrate is measured as an indication of amylase activity in the specimen. The residue may be amylose, amylopectin, starch, starch hydrolyzate, an etherified reducing terminal, an esterified reducing terminal, gluconolactone or a gluconic acid residue or its derivative. Decomposed substrate assay may be effected by contacting the same with maltose dehydrogenase and NAD or NADP, whereupon the assay is performed by measuring the amount of reduced NAD or reduced NADP, by reacting the same with reduced-form hydrogen transport colorimetric reaction reagent. This reagent may be a tetrazolium salt and diaphorase, or tetrazolium salt and phenazinemethosulfate. To remove pre-existing glucose and maltose present in the specimen, the specimen may be pretreated with alpha-glucosidase or kinase in the presence of Mg.sup.++ and ATP, the kinase being for example hexokinase. The preferred maltose dehydrogenase is produced by culturing Bacillus megaterium B-0779 FERM-P No. 5662.