Abstract 1. 1. Phenylalanine hydroxylase ( l-phenylalanine, tetrahydropteridine: O 2 oxidoreductase (4-hydroxylating), EC 220.127.116.11) has been purified up to 424-fold from the liver of the Cynomolgus monkey ( Macaca irus). This was achieved after stabilization of the enzyme during extraction and chromatography. 2. 2. In the final stages of purification two fractions have been separated, both of which are essential for full activity. 3. 3. Conditions for maximal activity have been established and preliminary kinetic studies have been performed. The optimum pH was found to be 7.6. The K m for l-phenylalanine was found to be 5.7·10 −4 M and 8.5·10 −5 M for 2-amino-6,7-dimethyl-4-hydroxy-5,6,7,8-tetrahydropteridine hydrochloride.