Irradiation of protein free DNA with 1 and 2 Mev electrons produces profound changes in the physico-chemical properties of the macro-molecule. Variations in the molecular weight, radius of gyration, viscosity and solubility indicate that 2 main types of damage are produced. Firstly, ionization clusters give rise to breaks in both chains of the twin helix with an efficiency of 800 ev per break and such damage is not affected by the presence of oxygen. Secondly, at the site of a break in one strand of the helix only, an active center is produced capable of reacting with another active center in another molecule to produce a crosslink. Oxygen reduces the amount of crosslinking by combining with the active center to render it inactive. In vacuo one crosslink is produced for every 1200 ev deposited.