The effect of low dose-rate total body irradation (TBI) on hemopoietic and nonhemopoietic lethality has been studied in BALB/c mice using dose-rates ranging from 25 to 1 cGy/min. Deaths were scored at 10 days, 30 days, and one year after irradiation, and dose-response curves were constructed to determine the dose-rate dependence of deaths from the gastrointestinal syndrome, hemopoietic syndrome, and late lethal syndrome(s), respectively. A plot of the LD50S for each of these lethal syndromes versus dose-rate showed the dose-rate dependence for late lethality to be somewhat greater than that for gut death, but both of these endpoints were markedly more dose-rate dependent than was hemopoietic lethality, particularly at dose rates less than 5 cGy/min. To determine which late responding normal tissues might be critical for low dose-rate TBI, complete necropsies were performed on all mice dying later than 60 days after irradiation and on all mice surviving at one year; all tissues were examined histologically. Morphologic evidence of radiation injury was present in only three tissues, lung (fibrosis and scarring) kidney (tubule depletion), and liver (presence of mitoses). Subjectively, the lung changes were most severe up to 9 months while kidney changes became more prominent after this time, suggesting that late death after low dose-rate TBI may not be entirely attributable to lung injury. However, regardless of which late responding normal tissue is dose-limiting, it is clear that low dose-rate TBI preferentially spares these tissues compared with hemopoietic stem cells.