A fast and sensitive method for determination of the antineoplastic drug methotrexate in untreated serum and urine is described. Reversed-phase liquid chromatography with sodium dodecyl sulfate (SDS) micellar mobile phase has been used with direct sample injection. Changes in mobile phase variables such as SDS concentration and/or pH profoundly affected drug retention. Recovery was quantitative and the detection limit (90 nM for a 20 microL sample size) was below the range normally monitored. Multichannel UV detection provided strong evidence regarding peak purity and peak identity.