The significance of cell cycle time in relation to radiation sensitivity under continuous exposure is a subject of considerable interest in radiobiology. Lamerton and Lord (1) have reported that for rats the epithelium of the small intestine, which has a short cell cycle, showed a much greater tolerance to continuous irradiation than the bone marrow, for which the intermitotic period is longer. These same workers have also described preliminary results with two transplantable mouse tumors, which indicate that the tumor with the shorter cell cycle was the more resistant to continuous irradiation (1). Lamerton pointed out that these results were consistent with the attractive theory that the shorter the cell cycle, the less is the effect of a given dose rate. The present communication reports an inverse relationship between sensitivity to continuous exposure and length of cell cycle in seedlings of Vicia faba. Seedlings of Vicia faba offer, in many ways, a unique opportunity to study the effects of irradiation on a relatively simple organized system. The patterns of root growth at 19?C during continuous exposure at dose rates between 0.32 and 11.4 rads/hour have been described in detail by Hall (2). Mathematical models have been suggested for the relationship between division and differentiation in the normal and irradiated meristem (3, 4), and computer calculations, based on these models, of the cell population kinetics under continuous irradiation have produced theoretical curves of root growth very similar to those observed experimentally (5). These theoretical calculations have predicted the same growth pattern for a given dose per cell cycle independent of cycle time, in keeping with the relationship suggested by Lamerton. Evans and Savage (6) have reported that the average intermitotic period of the root meristem cells of Vicia cultured at 12?C is about twice that for roots grown at 19?C. Thus, by the use of the two culture temperatures, the dose response could be