ABSTRACT The present study was designed to reproduce in the! laboratory an acute exposure of mice to early fallout (2 days old) such as might result'from a land-based nuclear detonation. Biological data werg obtained on the uptake, distribution, and retention of the inhaled fallout simulant. Further, as a basis for comparingithe effects of different types of simulants, the metabolig*behavior of this dry-particle fallout simulant of limited golubility was comparedto that of two previously studied fallout simulants; an ionic liquid aerosol and a mud-slurry aerosol. The simulant was also adminis- tered by gavage to provide data on uptake of fallout byiinges- tion as compared with inhalation, From these data, an evaluation was madg of the radiation dose to individual tissues from inhaled fallout as compared to the concomitant external radiation dose that the animals would receive if expoged to the same airborne sjmulant.