WE have previously reported that the infectious nucleic acids obtained from different strains of tobacco mosaic virus have the same sensitivity to inactivation with ultra-violet light1. The intact strains, however, differ in this regard2. Strain U2 has the same sensitivity as the infectious nucleic acid, whereas strain U1 is 5½ times more resistant. It was therefore concluded that the type of combination between protein and nucleic acid that exists in the U1 strain affords appreciable protection from the damaging effects of ultra-violet light.