Letheen Broth Base is used with Polysorbate 80 for the testing of quaternary ammonium compounds for antimicrobial acitivity. Polysorbate 80 is not supplied. In 1948, Weber and Black described the value of a highly nutitional solid medium containing neutralizing agents for quaternary ammonium compounds for sanitizers (1). The addition of Lecithin and Polysorbate 80 to Tryptone Gluse Extract (TGE) Agar resulted in a medium that effectively neutralizes quaternary ammonium compounds while testing germicidal activity. Letheen Agar is a modification of TGE Agar with the addition of Lecithin and Polysorbate 80. Letheen Base was developed as a subculture medium for the neutralization of quaternary ammonium compounds in disinfectant testing. Quisno, Gibby and Foter discovered that adding lecithin and Polysorbate 80 to FDA Broth results in a medium that neutralized high concentrations of quaternary ammonium salts (2). he resulting medium, termed Letheen (a combination of Lecithin and Tween) was