Both the separation and determination of aspoxicillin in human plasma by micellar electrokinetic chromatography (MEKC) were investigated. Selectivity in the separation of seven penicillin antibiotics was improved by using MEKC in comparison with capillary zone electrophoresis. Plasma proteins, which might interfere with drug analysis in conventional chromatography, were solubilized by the micelles employed in MEKC and eluted later than the drugs. This permitted the determination of the drugs in plasma by a direct sample injection method. One analysis of a plasma sample was performed within ca. 20 min without pretreatment. Good linearity and recovery were also obtained in the range of plasma levels usually encountered in clinical analysis with a correlation coefficient r = 0.999 and 94-104% recovery. The limit of detection for aspoxicillin was 1.3 micrograms ml-1 at a signal-to-noise ratio of 3.