Norfloxacin produced a reliably bactericidal effect at concentrations from 3 to 90 mg/l against urine pathogens suspended in human urine. These included Enterobacteriaceae, Pseudomonas aeruginosa, Streptococcus faecalis and Staphylococci. Resistant mutants of Staphylococcus aureus were isolated on two occasions (out of 33 experiments). At high concentrations (c. 90 mg/l) the activity was less but this was probably due to the need for a low pH to dissolve the antibiotic. The pH for optimum activity of norfloxacin in urine was 7.5 to 8.0. Human blood had little effect on the bactericidal activity. Compared to other antibiotics, the rate of killing of cultures in urine was second only to gentamicin.