1. 1. The effect of DNA degradation products on the cleavage of Arbacia eggs was studied. Some experiments were performed in summer when eggs were scarce and ovaries undersized. A locally made Arbacia DNA and two commercial DNAs were used, one from thymus, fibrous and highly polymerized, the other from salmon, a powder. 2. 2. The undegraded DNAs were stimulating except the highly polymerized material which was inhibitory probably due to interference with surface activities of the eggs. 3. 3. Autoclaved salmon DNA was inhibitory presumably because of split inorganic PO 4; autoclaved Arbacia DNA was stimulating and the highly polymerized autoclaved material was inactive. 4. 4. Autoclaved DNA solutions were passed through a cellulose ECTEOLA column and fractions eluted with 0.1–2.0 M NaCl solutions (the main constituents of such fractions are polynucleotides). 5. 5. Fractions of autoclaved salmon DNA were inactive but the Arbacia DNA fractions were stimulating. 6. 6. Crystalline DNA-ase was used to digest the three DNAs and fractions of the digest were collected from ECTEOLA column as in no. 4. 7. 7. Fractions of salmon DNA digest were inactive whereas fractions of Arbacia and highly polymerized digests were very active, sometimes raising the rate of division to 100 per cent as compared with the very low summer rate of about 5 per cent. 8. 8. Differences in the actions of the three DNAs were explained on the basis of their degree of polymerization. The pronounced stimulation of cleavage of summer eggs was explained as correction of some deficiency in polynucleotides. Fractionation selected biologically active material (polynucleotides) with specific properties.