SUMMARY

Some damage caused in tobacco necrosis virus by u.v. radiation could be repaired in darkness in Chenopodium amaranticolor (dark reactivation) but not in French bean or in tobacco. By contrast, photoreactivation of the irradiated virus was observed in French bean and in tobacco but not in Chenopodium. The kind of damage in u.v.-irradiated virus that is susceptible to repair by photoreactivation appears to be repaired by dark reactivation i Chenopodium. In conditions without evidence of any repair, the quantum yield for inactiva-tion of the nucleic acid inside the virus is about 6"5 x IO-*, and the amount of radiation energy that must be absorbed by the nucleic acid to reduce infecti-vity to 5o ~o is about 0"3 J/rag