The study of the influence of the 3′-adenylic acid on cell division has shown that it has an inhibiting effect. This substance also produces various types of chromosomal aberrations, especially the picnosis phenomenon. By studying the effect of adenine, adenosine and 3′-adenylic acid on the mitotic cycle inVicia faba, adenine and the 3′-adenylic acid have been found to considerably increase the length of the mitotic cycle. The authors consider that the strong effect of the 3′-adenylic acid upon the nucleus and the chromosomes is probably due to the DNA depolimerization (fact pointed out by the picnosis phenomenon). This also contributes to the blocking of the macromolecules synthesis by the deoxyribonucleic acid.