Thomas, Rostand and Gregoire1 observed that the puncture with a needle which had been dipped in a relatively pure solution of ribonuclease inhibits the development of undivided frog's eggs and prevents cellular division. They also showed that no such effect is obtained with deoxyribonuclease, nor with pepsin or trypsin used under the same conditions. We thought it of interest to repeat these observations with highly purified enzymatic preparations, taking advantage of the fact that only reduced ribonuclease has enzymatic activity2. © 1954 Nature Publishing Group.