Guinea pigs were exposed via the respiratory tract for 12 hr to 85Kr concentrations ranging from 54 to 520 μCi/cm3 and observed for 60 days or until death, whichever occurred first. The 30-day median lethal exposure was 4450 (μGi/cm3) hr which corresponds to estimated doses of 316 rads to the bone marrow, 340 rads to the whole body, and 7876 rads to the lungs. The marrow and whole-body doses, times of death and the gross clinical and necropsy observations were consistent with death by the “hematopoietic syndrome” but lung injury was dominant in animals that survived for more than 30 days. The whole-body and marrow doses delivered at the median lethal exposure and the observed times of death are similar to those reported in X- or gamma-radiation lethality studies.