A simple, rapid and sensitive isocratic reversed phase HPLC method with UV detection using internal standard has been developed and validated for simultaneous determination of amoxicillin and clavulanic acid in human plasma. The assay enables the measurement of amoxicillin and clavulanic acid for therapeutic drug monitoring with a minimum quantification limit of 15 and 30ngml⁻¹, respectively. The method involves simple, one-step extraction procedure and analytical recovery was complete. The separation was carried out in reversed-phase conditions using a Chromolith Performance (RP-18e, 100mm×4.6mm) column with an isocratic mobile phase consisting of 0.02M disodium hydrogen phosphate buffer–methanol (96:4, v/v) adjusted to pH 3.0. The wavelength was set at 228nm. The coefficients of variation for inter-day and intra-day assay were found to be less than 9.0%.