In 20 patients undergoing cataract extraction, we measured pefloxacin concentrations in the eye. After a 1 h infusion of pefloxacin 400 mg iv, the mean pefloxacin concentrations 2, 6, 12 and 24 h post-infusion were 0.75, 1.45, 1.04 and 0.86 mg/l, respectively. The simultaneous mean pefloxacin concentrations in the lens were 0.09, 0.20, 0.26 and 0.43 mg/l, respectively. The adequate penetration of pefloxacin into uninfected aqueous humour and lens suggests that this antibiotic may be effective in the treatment of certain bacterial endophthalmic infections.