E. coli beta-galactosidase is a 464 kD tetramer. Each unit of beta-galactosidase consists of five domains, the third of which is the active site. It is an essential enzyme in cells. Deficiencies in this enzyme can result in galactosialidosis or Morquio B syndrome. In E. coli, beta-galactosidase is produced by the activation of LacZ operon. Detection of LacZ expression has become routine to the point of detection of as few as 5 copies of _-galactosidase per cell. This kit uses a fluorogenic galactosidase substrate that can sensitively distinguish LacZ+ vs. LacZ-cells. It can be used either for detecting galactosidase conjugates in ELISA type assay systems or for monitoring LacZ gene expression in cells. The galactosidase-cleaved product has Ex/Em = 490/520 nm that can be detected with most of fluorescence instruments equipped with a FITC filter set. This kit is optimized for flow cytometry applications.