Chameleon™ mEI Agar is a selective and differential, chromogenic medium used for the identification of Enterococcus. Levin, et al. developed a two-step membrane filter method to measure enterococci in fresh and marine recreational waters. Using mE Agar, the method required a 48-hour incubation and a transfer of the membrane to another substrate medium, Esculin Iron Agar, to differentiate enterococci. In 1997, the EPA improved the mE Agar formula by reducing the amount of triphenyltetrazolium chloride contained in the medium and also added the chromogen indoxyl-B-D-glucoside. The new medium, mEI Agar, was developed as a single-step procedure that does not require the transfer of the membrane to another substrate.• Product meets APHA, EPA, and SMWW reference methods and performance criteria.• mEI should be used by trained professionals within a laboratory setting. • Product is not intended for use in the diagnosis or treatment of disease or other human conditions.