The enzyme that catalyses the hydroxylation of the methyl group of p-cresol was purified from Pseudomonas putida. It has mol.wt. 115000 and appears to contain two subunits of equal molecular weight. One subunit is a c-type cytochrome and the other is a flavoprotein. Reduction of the cytochrome occurred on addition of substrate. The same enzyme catalyses both p-cresol hydroxylation and the further oxidation of the product, 4-hydroxybenzyl alcohol. The stoicheiometry of acceptor reduced per molecule of substrate oxidized is that for two dehydrogenation reactions. The Km for p-cresol is 7.3 x 10(-6) M and that for 4-hydroxybenzyl alcohol is 47.6 x 10(-6) M. The enzyme, which is assayed with phenazine methosulphate as electron acceptor, was stimulated by particulate material, which probably contains the acceptor in vivo.