Abstract Fusidic acid was determined in plasma by a high-performance liquid chromatographic method. Fusidic acid was extracted from plasma with acetonitrile that was salted out with ammonium sulfate. Prior to salting out cadmium sulfate was mixed with the acetonitrile-plasma mixture to help remove interfering constituents. A 150 mm × 4.6 mm column packed with 5-μm cyanopropyl stationary phase was used for chromatography. The mobile phase was acetonitrile-20 m M sodium dihydrogenphosphate (pH 3.50) (39:61, v/v). An ultraviolet-visible detector was set at 204 nm. The presence of water in the injection solvent had a significant effect on the fusidic acid peak height. A number of clinically important acquired immunodeficiency syndrome drugs did not interfere with the fusidic acid determination. The relative standard deviation varied between 0.99 and 7.8%. A limit of detection of 200 ng/ml was obtained for a 80-μl injection.