Aldol® 515 β-D-galactopyranoside is a chromo- and fluorigenici ndicator for β-galactosidase activity. The colorless product is cleaved by lactose-utilizing bacteria in liquid media or on agar plates and yields an intense red color. In addition, red fluorescence signals are generated if a fluorescence enhancer or a suitable matrix is present. Aldol® 518 β-D-galactopyranoside can also be used as detecting agent for β-galactosidase activity in reporter gene assays or for identification of positive transformants on agar plates when using the α-complementation approach. Color and fluorescence formation is due to on an intramolecular aldol condensation after enzymatic cleavage. The reaction proceeds equally well in the presence and absence of oxygen, i.e., in aerobic andanaerobic cultures. Well detectable color/fluorescence of bacterial colonies or liquid cultures is obtained within 16-48 h. In the presence of purified β-galactosidase, color and fluorescence develops within minutes to few hours.