Subcellular fractionation by differential centrifugation was used to study protocollagen proline hydroxylase (EC 1.14.11.2) localization from chick embryo liver. The fractions have been characterized by marker enzymes and electron microscopy. By these methods, it was observed that procollagen-proline-hydroxylase is concentrated in the microsomal fraction which is sedimented at 145 000 X g in 250 mM sucrose.