The use of high-dose methotrexate (MTX) therapy in cancer chemotherapy has intensified the need to monitor the purity of this drug. A high-pressure liquid chromatographic method, which utilizes ion-pairing on a mu-Bondapak C18 column, has been developed for this purpose. This method provides an efficient and highly sensitive method for detecting impurities in commercially available MTX. As has been demonstrated previously, patients receiving high-dose MTX therapy metabolize some of this drug to 7-hydroxymethotrexate (7-hydroxy-MTX), a derivative which is considerably less soluble than MTX itself. Consequently, 7-hydroxy-MTX may contribute to the renal toxicity commonly associated with high-dose therapy. An assay similar to the one described above is also applicable to this problem and can be used to assay MTX and 7-hydroxy-MTX directly in patients' urine.