Effects of 1000 R, whole-body X-irradiation on the proliferative cells of the mouse duodenal crypts, in the four phases of the generation cycle; namely, the DNA synthesis phase, S; the pre-mitotic gap, G2; the division phase or mitosis, M; and the pre-synthesis gap, G1. As pointed out by Whitmore and Till (1964) G1 and G2 are characterized only by the fact that no DNA synthesis is taking place in these phases.