[en] Inhalation of radionuclide-bearing particles remains one of the most intensely pursued problems concerning the nuclear industry. This route of entry is generally accepted as the most probable, in case of human exposure, with ingestion being the other prominent source of concern. Many laboratory investigations, such as those reported here, continue to evaluate the possible consequences that may present health problems to the public domain. Syrian hamsters of both sexes received either inhaled (INH) PuO₂/ZrO₂ particles, intravenous (IV) PuO₂/ZrO₂ microspheres, a combination of INH PuO₂/ZrO₂ particles and injected PuO₂/ZrO₂ microspheres, or no radionuclides (controls). The INH particles and IV microspheres were tagged with γ-emitting ⁵⁷Co to facilitate whole body counting and establishment of retention curves. Total lung burdens ranged from 8 nCi to 143 nCi. Significant numbers of primary lung tumors (5 to 50% per group) were induced in those animals that received INH exposures. Additional α radiation administered via Pu-laden IV microspheres had little or no effect on tumor production or nonneoplastic, degenerative changes in the respiratory tract