This book encompasses the technical details of phenylalanine ammonia -lyase (PAL) assay which serves as a quick easy method for detecting phenylketonuria (PKU), an inborn error of phenylalanine metabolism in neonates. The protocol of extraction, immobilization of PAL from a higher plant (Cucumis sativus L.) and its application in assessing the activity of a hepatic key enzyme of the malady, phenylalanine hydroxylase (PAH) in the animal experimental system (Wistar Rat Model) are explained. The principle of population screening is to identify persons with biologic markers of the disease. A simple test is imperative to identify the biomarkers of the disease. The novel PAL assays developed in the present research are valid on the pCPA-administered rat model system which can be extended to human phenylketonurics. The book would be immensely useful for clinical biochemists, geneticists and enzymologists.