PURPOSE: To assay a fused protein content in a short time by diluting the mixture of the fused protein with proteolytic products with bovine serum albumin and subsequently reacting the diluted mixture with α-amylase.

CONSTITUTION: This method for assaying a fused protein content comprises diluting (A) the mixture of the fused protein with proteolytic products, such as a supernatant obtained after the centrifugal separation of a fermented solution, with a bovine serum albumin into a volume of about 100 times, mixing a part of (B) the diluted mixture with (C) an α-amylase solution originated from swine pancreases, incubating the mixture at about 37°C for ≥30 min, adding (E) an α-amylase reagent to (D) the incubated reaction product, measuring the mixture with a spectrophotometer to obtain the residual activity value (F) of the component C, mixing the other part of the component B with the component E, adding the component C to the mixture, assaying the mixture with a spectrophotometer to obtain (G) a reaction rate, and subsequently comparing the activity value F with the rate G to assay the content of the fused protein the component A.