The antibacterial activity of hexamine and two of its organic acid salts was compared by continuous turbidimetric monitoring of static cultures exposed to the drugs and in an in vitro model of the treatment of bacterial cystitis. At pH 5.5, concentrations of 32 to 125 mg hexamine per 1 caused some inhibition of bacterial growth, but 250 to 500 mg/l were needed to suppress growth overnight. Hexamine hippurate was found to be less active than hexamine itself, whereas hexamine mandelate was as active as the parent compound. The antibacterial effect was not inoculum dependent over the range 106 to 108 bacteria per ml, but the activity observed in pooled urine was rather less than that found in broth at the same pH. When a dense bacterial culture was exposed to changing concentrations of hexamine compounds in the bladder model, a 1 g dose infused over a 12-hour period suppressed bacterial growth for 16 to 20 hours. No systematic difference was found in the activity of the three hexamine compounds, but a similar effect was achieved by a very much lower concentration of ampicillin.