Exposure of cellsin vitroto X ray doses of 2500 r, 5000 r, and 10,000 r leads to the breakdown of cells soon after irradiation. The greatest number of degenerate cells are observed 3 hours after exposure.Cultures in which mitotic activity was greatly reduced before exposure respond in the same manner as normal cultures.The time at which degenerate cells appear, and the absence of mitotic recovery during the period of observation, indicate that the degeneration seen after these dosages is due to a breakdown of cells in the resting condition.This in contrast with the results following exposure of similar cultures to a dose range of 100–1000 r.