5-Iodo-3-indolyl b-D-galactopyranoside is a chromogenic substrate that can be used in food safety testing. It is also a ligand for the peroxidase enzyme and can be used in biochemical and biological studies. 5-Iodo-3-indolyl b-D-galactopyranoside has been shown to be an excellent substrate for chemiluminescence reactions, which are useful in environmental testing.