A new enzymatic assay method for ornithine carbamoyltransferase, OCT, activity was developed using ornithine:2-oxo-acid aminotransferase, OKAT, and 1-pyrroline-5-carboxylate dehydrogenase, P5CDH. The method is based on the reaction of L-ornithine formation from L-citrulline by OCT. The OCT activity was measured spectrophotometrically at 340 nm and the minimum detectable activity was about 4 units/L of serum. Results obtained by this method and by the chemical method correlated well. This method is superior to the chemical method, as it does not require boiling of the reaction mixture nor removal of interfering substances in the serum.