Hydroxyurea, a compound which inhibits the growth of rapidly proliferating tissues in animals and man, is known to inhibit DNA synthesis, and studies in crude systems have indicated that it does so by preventing the reduction of ribonucleotides to deoxyribonucleotides.

This study demonstrates hydroxyurea-induced inhibition of the reduction of ribonucleotides by a highly purified enzyme system from Escherichia coli B. The inhibition (70% at 3 × 10^-3 m under standard incubation conditions) is dependent on the concentration of hydroxyurea and on the duration of exposure of the enzyme to hydroxyurea. The effect of hydroxyurea is specific for protein B2 of the ribonucleoside diphosphate reductase system. The inhibition of enzymatic activity is apparently irreversible, although there is negligible irreversible binding of ¹⁴C-labeled hydroxyurea to the enzyme protein.