Abstract
The relations between dose, survival, and survival-time after exposure to x- or γ-rays of doses ranging from 20 to 200 000 R were examined with rat thymocytes in vitro. The shape of the curve of survival against survival-time suggested that the thymocyte population consisted mainly of two kinds of cells, differing in the time course of development of radiation damage. Over the dose-range 760–6000 R, survival and survival-time were independent of dose. The causes leading to the cell death are probably different below and above 6000 R.