A new reverse-phase high-performance liquid chromatographic (HPLC) procedure has been developed for the quantitation of vancomycin and its crystalline degradation product, CDP-1. The new HPLC procedure involves a liquid-liquid extraction pretreatment procedure and an HPLC internal standard, vitamin B12. The new HPLC procedure was used to measure 50 renally impaired patient samples. Samples were than analyzed by the monoclonal antibody Emit vancomycin assay, and the polyclonal antibody fluorescence polarization immunoassay (FPIA) vancomycin assay, and the results were compared. The Emit vancomycin assay correlated well with HPLC, but FPIA quantitated higher than Emit and HPLC. We conclude that the polyclonal antibody FPIA assay detects CDP-1 in the samples of renally impaired patients, accounting for an average of 10% of the FPIA/Emit bias, and should therefore be used with caution when monitoring vancomycin levels in renally impaired patients.