AbstractThe rapid decrease in extinction of deoxyribonucleic acid in the first moments of irradiation with U.V. light (2537 Å) is due to the conversion of the pyrimidines into low‐absorbing substances. Thymine in this natural polymer reacts according to the so‐called “first irreversible reaction” shown by the free bases in solution. This could be demonstrated by means of a peculiar property of the irradiation product of thymine. Cytosine is supposed to react in the same way. The importance of this finding with respect to the lethal action of U.V. light on micro‐organisms is