Recent results have provided some interesting information concerning the mechanism of AdoCbl dependent ribonucleotide reductase (RTPR). The 3' carbon-hydrogen bond of (3'-3H) UTP is cleaved during its conversion to 2'dUTP catalyzed by Lactobacillus leichmannii ribonucleotide reductase. A selection against 3H of approximately 1.8 is observed on this reduction. During the course of this reaction, no 3H is released to the solvent, and no 3H is recovered in the reisolated cofactor. Incubation of (3'-2H) UTP with enzyme resulted in production of 2'-deoxy-(3'-2H) UTP. Incubation of RTPR 2'-chloro-2'-deoxyuridine 5'-triphosphate (2'ClUTP) has also been studied. This reductase catalyzed the release of 3H, uracil and tripolyphosphate from (3'-3H) ClUTP. The amount of 3H volatilized is approximately equivalent to the amount of AdoCbl present and is accompanied by production of 5'-deoxyadenosine.