For differentiation of the members of Enterobacteriaceae on the basis of potassium cyanide tolerance. Enterobacteriaceae are large family of gram negative, rod shaped bacteria that includes, along with many harmless symbionts, many of the more familiar pathogens, such as Salmonella, Escherichia coli, Yersinia pestis, Shigella. One of the many tests employed for the identification of bacteria includes the ability of an organism to grow in the presence of cyanide. Potassium Cyanide Broth Base is used for the differentiation of members of Enterobacteriaceae on the basis of potassium cyanide tolerance. Potassium Cyanide Broth Base was originally formulated by Moeller and Kauffman and Moeller. Proteose peptone provides nitrogenous compounds, sulphur, trace elements essential for growth. Monopotassium Phosphate acts as a buffering agent. Sodium chloride maintains osmotic equilibrium. Potassium cyanide inhibits many bacteria including Salmonella, Shigella and Escherichia, while members of the Klebsiella, Citrobacter, and Proteus groups grow well. Potassium cyanide medium usually remains stable for upto 4 weeks at 4°C. An elevated temperature leads to accelerated deterioration of KCN in the medium or evaporation of cyanide. The KCN should be destroyed before autoclaving by the addition of a crystal of ferric sulphate and 0.1 ml of 40% potassium hydroxide per tube. A heavy inoculum should be avoided while utilizing KCN Broth as the inoculum itself causes turbidity or sediment at the bottom of the tube, which may be interpreted as false positive result. Reactions observed in KCN Broth are not sufficient to speciate organism; additional biochemical and serological tests are required.