A sensitive and reproducible method for the measurement of moxalactam in plasma and cerebrospinal fluid is described. Plasma proteins were removed by precipitation with ice-cold methanol at pH 5.6 and centrifugation. The supernatant was analysed by HPLC on a mu-Bondapack/phenyl column, with a mobile phase of acetonitrile/water/PIC Reagent A (20/80/1), and detection at 280 nm. The calibration curve was linear for plasma concentrations from 10 micrograms/ml to 60 micrograms/ml. Reproducibility was 4.7% (coefficient of variation) for within-day analysis and 13.8% for day-to-day analysis. Plasma concentrations in 9 moxalactam-treated patients with severe infections ranged from 0.9 micrograms/ml to 409 micrograms/ml. Individual pharmacokinetic parameters were calculated using a personal computer. In selected cases moxalactam concentrations were also determined in cerebrospinal fluid and tracheal aspirates.