[en] The carcinogenicity of inhaled, submicron-sized, high-fired 239-plutonium dioxide and beryllium oxide particles was examined in over 600 rats, following single nose-only exposure to initial alveolar depositions of 1.0-91 μg beryllium and of 4-180 nCi ²³⁹Pu. The alveolar half-life of beryllium in the lung was 325 days. Only one of 184 rats exposed to beryllium-only developed a lung tumor during the 2 yr observation period following exposure. None of 128 unexposed rats developed lung tumors. The incidences of lung tumors were 10% at an initial alveolar deposition of 5 nCi²³⁹Pu, 37% at 45 nCi and 53% at 180 nCi. The alveolar clearance of ²³⁹Pu was decreased and the translocation of ²³⁹Pu to thoracic lymph nodes was increased in rats given Pu0₂ after exposure to beryllium oxide. The incidences of lung tumors following combined exposure were 15% at initial alveolar depositions of 7.6 nCi²³⁹Pu and 14 μg beryllium, 38% at 76 nCi²³⁹Pu and 14 μg beryllium, 12% at 3.8 nCi²³⁹Pu and 91 μg beryllium, and 58% at 59 nCi²³⁹Pu and 91 μg beryllium. It was concluded that previous exposure to beryllium oxide did not significantly change the incidence of lung tumors following a subsequent exposure to ²³⁹Pu0₂, despite the effects of beryllium on the alveolar clearance of ²³⁹Pu. (author)