A method is described for the assay of PALA [a potential antineoplastic drug] in plasma or urine, based on inhibition of partially purified aspartate carbamoyltransferase [EC 2.1.3.2] from rat liver. Concentrations of PALA as low as 0.1 .mu.g/ml can be detected. Plasma disappearance and urinary excretion curves from 1 patient are shown as examples of the application of the method.