Thirty-three patients requiring prolonged drainage of the urinary bladder in 35 separate instances were treated with a constant bladder irrigation of a dilute solution of neomycin sulfate and polymyxin B sulfate. This system was highly effective in preventing the acquisition of bacteriuria caused by gram-negative rod forms in patients with initially sterile urine if the catheter remained in place for ten days or less, but it was considerably less effective in preventing acquired bacteriuria when catheter drainage was required for periods in excess of ten days. The bladder rinse was not effective in eradicating preexisting bacteriuria. Although complications of therapy were minimal, prolonged irrigation with neomycin-polymyxin B solution seemed to predispose to acquired bacteriuria or superinfection withProteusspecies and enterococci which were usually resistant to at least one of these antibiotics.