Abstract—–Nicotiana tabacumvar.Xanthin.c. plants placed in the dark lose their ability to photoreactivate u.v.‐irradiated tobacco mosaic virus RNA over a course of approximately 1 week. When such plants are returned to the light, they recover their photoreactivation ability. The recovery occurs after a lag of at least 3 hr and is complete in 12–24 hr. Three hours or less of cool‐white illumination (2400 ft‐c) are necessary to induce recovery. Blue light is effective in inducing recovery; green light is less effective; red and near‐u.v. are