A radioimmunoassay procedure has been developed for the direct measurement of methotrexate (MTX) in plasma, serum, cerebrospinal fluid, or urine samples. The assay is sensitive to levels of at least 100 pg of MTX and is highly specific for MTX in the presence of folinic acid (citrovorum factor), folic acid, tetrahydrogolic acid, and other folate analogs and known metabolites. Results from this procedure have been compared with those obtained with a spectrofluorimetric method, utilizing the plasma of cancer patients undergoing high-dose MTX treatment with citrovorum factor rescue. Results indicate that the method should be usful in the future in assisting individualization of dosage regimens and in the study of the pharmacokinetics and metabolism of MTX in cancer patients.