Ritonavir is an HIV-1 protease inhibitor that is often used to improve the systemic availability of concurrently administered protease inhibitors by impairing their metabolism through cytochrome P450 (CYP) 3A4. Pharmacodynamic relationships between plasma ritonavir concentrations and efficacy and toxicity have also been described. To date, published high-performance liquid chromatographic (HPLC) methods for the determination of ritonavir in human plasma are often complex, requiring the use of a buffered mobile phase that contains amine-modifiers (i.e. diethylamine, triethylamine). In the method herein, ritonavir was precipitated with acetonitrile plus barium hydroxide and zinc sulphate. Chromatographic separation was accomplished using a C-18 base-deactivated (250 x 4.6 mm I.D., 5 atm particle size) analytic column with a mobile phase composed of acetonitrile:water (52:48, v/v). Quantification was performed at 239 nm. Calibration curves were linear from 0.5-25 microg/ml (R2 > 0.999); percent errors, as a measure of accuracy, were < 12.7%. Intra- and inter-assay relative standard deviations (RSD) were below 12.8%. This method provides a rapid and simple means for the accurate and precise analysis of ritonavir in human plasma. Furthermore, the assay requires neither the use of a buffered mobile phase adjusted to a specific pH, nor the addition of amine modifiers. This method has been successfully used to determine plasma ritonavir concentrations in drug interaction studies.