Fluorescein di(β-D-galactopyranoside) is a fluorogenic substrate for β-galactosidase (λ_ex=485 nm, λ_em=535 nm). In Vitro: The fluorescence produced by Fluorescein di(β-D-galactopyranoside) increases in a time- and dose-dependent manner. The level of fluorescence produced by the double-substrate method is much lower than that by the Fluorescein di(β-D-galactopyranoside) method. Results show that the fluorescein produced by Fluorescein di(β-D-galactopyranoside) in Hs68 cells is proportional to the number of passages^[1].