A method for the determination of a non-nucleoside HIV-1 reverse transcriptase inhibitor in human plasma is described. Plasma samples are extracted using phenyl solid-phase extraction columns. The extract is analyzed by high-performance liquid chromatography with a polybutadiene-coated alumina column and a mobile phase of methanol-0.025 M pH 8 dibasic sodium phosphate buffer (1:1, v/v). Detection is based on ultraviolet absorbance at 326 nm. The assay was validated in the concentration range 10-500 ng/ml when 1-ml aliquots of plasma are extracted. The assay has been utilized to support human pharmacokinetic studies.