This kit provides a quantitative, rapid, simple, and sensitive method for the determination of mannose-6-phosphate (M6P) content. Mannose-6-phosphate is converted to fructose-6-phosphate by phosphomannose isomerase; fructose-6-phosphate is then converted by an enzyme complex comprising phosphoglucose isomerase and glucose-6-phosphate dehydrogenase, leading to the conversion of NADP⁺ to NADPH. The M6P content is calculated by measuring the increase in NADPH at 340 nm.