Binding of iron with the molecule of prolyl hydroxylase was studied using 59Fe. The label was administered into one-day-old rats and within 36 hours the enzyme was isolated from skin. After the final step of purification (DEAE cellulose) 59Fe was not found in fractions of prolyl hydroxylase. The purity of the enzyme preparations was controlled by polyacrylamide gel disc electrophoresis. The data obtained suggest that Fe ions are not inherent in the enzyme molecule.