To provide an assay for identifying a phosphatase inhibitor.

This method for identifying an acting agent for inhibiting the catalytic activity of PTPbr7 comprises a process of admixing a certain amount of PTPbr7 in an assay buffer containing a reducing agent, a certain amount of a substrate which can be dephosphorylated by the PTPbr7 and a certain amount of a active agent for a test as a first cocktail; a process of preparing a second cocktail containing all of the components in the first cocktail except for the active agent for the test; a process for incubating the first cocktail and second cocktail under a suitable condition enabling the substantial extent of the dephosphorylation of the substrate by the PTPbr7; a process for terminating the incubation; a process for determining the extent of dephosphrylation in each of the cocktails; and comparing the extents of the dephosphorylation, and the PTPbr7 inhibitor is the active agent for the test, generating a lower dephosphorylation in its presence than that in its absence.