This article reports a modified liquid chromatographic fluorescence assay that has proved in our laboratory to give robust performance for the purposes of therapeutic drug monitoring. The sample preparation involves extraction of alkalinized plasma into 1-pentanol/chloroform and back extraction into HCl. Acceptable reproducibility and accuracy data are presented over the concentration range normally encountered in patient samples. The detection limit (25 ng/ml) is approximately one-tenth the lowest calibration standard used in our laboratory for quantitating patient pre-dose specimens, making the method applicable to pharmacokinetic studies.