A simple isocratic HPLC procedure for the analysis of cocaine in plasma, with or without an internal standard, is described for the first time. Basified plasma was extracted in ether, re-extracted in acetic acid, which was subsequently basified prior to the final extraction in n-hexane. The hexane extract was evaporated to dryness, reconstituted in the mobile phase and then chromatographed. A reverse-phase micro-particulate C-18 column, a pre-column, and a UV detector set at 232 nm were used. A mobile phase containing 75% methanol and 25% 0.05 mol/L potassium phosphate buffer (pH 6.6) was used at a flow rate of 0.8 mL/min. Cocaine in the range of 20 to 500 ng/mL in plasma was determined on the basis of (a) peak height and (b) ratio of peak height to that of tetracaine internal standard. On either basis a linear regression on concentration was determined. The correlation coefficients (r) were 0.993 and 0.988 for (a) and (b) respectively. Twenty-two commonly used drugs were examined for interference. Eight drugs were considered candidates for potential interference with cocaine; lidocaine and droperidol were found to interfere in actual patients' samples. Only meperidine was found to interfere with the internal standard. Cocaine was determined in plasma from patients who received cocaine and other drugs.