This medium is recommended for determination of bacterial activity of quaternary ammonium compounds using Escherichia coli or Staphylococcus aureus. The word Letheen represents a combination of lecithin and polysorbate (tween) 80. Lecithin and polysorbate 80 to the medium enables the recovery of bacteria from materials containing residues of disinfectant compounds or preservatives used in cosmetics. It is available in 100 ml quantity.