Einsteinium-253 nitrate was administered as an aerosol to male Wistar rats and the long-term biological effects were followed for the animals' life span. Lung was the major target organ for absorbed radiation dose and tumor induction, in agreement with results for other inhaled transuranic nitrates. The earlier finding of a high incidence of bone tumors following intratracheal instillation of 253ESCl3 was not confirmed in the present study with inhaled 253ES(NO3)3 The reason for the difference in bone tumor production is believed to be related to the different acute toxicities of intratracheally instilled and inhaled 253ES. Intratracheally instilled 253EsCI3 was less acutely toxic (only a single lobe or one-half the lung was irradiated); thus, a larger more tumorigenic dose could be translocated to bone without shortening the life span to the extent that bone tumors could not be expressed. The radiation dose from inhaled 253Es(NO3)3 was uniformly spread throughout both lungs and early death due to a generalized radiation pneumonitis precluded the development of long-term effects in bone.