As part of a programme to establish the relative merits of antiseptics that are used as bladder instillations to control urinary tract infections in patients with indwelling catheters, the activity of five such formulations were tested against dense (10(9) cfu/cm2) mixed biofilms composed of Citrobacter diversus, Pseudomonas aeruginosa and Enterococcus faecalis growing on silicone discs. All three species were resistant to chlorhexidine (200 mg/l) and povidone-iodine (1% v/v) in the biofilm mode of growth, whereas these agents rapidly eliminated viable cells from urine suspensions of the mixed community. Lactic acid (1% v/v) produced a 1 log reduction of the biofilm population within 30 min of exposure. The mandelic acid (1% w/v) and mandelic acid (0.5% w/v)/lactic acid (0.5% v/v) mixture proved to be the most effective in eliminating the biofilm organisms. It is suggested that these latter solutions should now be tested for efficacy in bladder washouts against urinary tract infections in catheterized patients.