A 13-fold increase was observed in the specific activity of ribonucleotide reductase in the spleen of mice injected with a murine leukemia virus. There was an absolute requirement for a hydrogen donor, dithiothreitol. The ribonucleoside diphosphates were preferred substrates. The reduction rate was affected by the presence of nucleoside triphosphates. Ribonucleotide reductase is present in acute and chronic myelocytic leukemia leukocytes. Normal and chronic lymphocytic leukemia leukocytes showed no significant activities. Coenzyme B12 does not stimulate the enzyme from megaloblastic bone marrow