PYRIMIDINES halogenated in position 5 have been shown to sensitize bacteria¹, bacteriophages² and mammalian cells³ to ultra-violet irradiation. In all these cases substitution of thymine, a specific constituent of deoxy-ribonucleic acid (DNA), by its halogenated analogues is implicated as the primary cause of increased sensitivity. This, together with the fact that one of the analogues, 5-bromouracil, is a known mutagenic agent in bacteria⁴ and bacteriophages⁵, strongly suggests that genetic material, that is, DNA, is the primary target of irradiation.