Methenamine hippurate is a urinary antibacterial agent which releases formaldehyde and is excreted in the urine. Methenamine hippurate has a demonstrable activity in vitro at pH 5 to 6 against Escherichia coli , Staphylococcus aureus , Aerobacter aeroaenes , Pseudomonas aeruginosa and Proteus vulgaris , and a wide range of action against Klebsiella pneumoniae , Ps. aeruginosa , E. coli , Staph. aureus and Streptococcus haemolytic at pH of 7.4. This action is more effective than that of Methenamine alone, of Hippuric acid alone or of Sulphadiazine.