PURPOSE: To obtain a substrate for a β-galactosidase, related to a liposome immunoassay, increased in sensitivity and stable in an aqueous buffer by preparing the substrate for the β-galactosidase from a composition represented by a specific general formula.

CONSTITUTION: This substrate for a β-galactosidase comprising a composition represented by formula I {X is a halogen atom, nitro group or hydrogen atom; Y is a halogen atom, a lower alkyl group or hydrogen atom; W is hydrogen atom or a lower alkyl group; Z is nitro group or a group represented by an aromatic ring structural formula represented by formula II [(a) is a lower alkyl group or hydrogen atom; (b) is a halogen atom or hydrogen atom; (c) is a lower alkyl group or hydrogen atom; (d) is SO3Na; X is not nitro group when Y is hydrogen atom] is prepared.