A method was developed to determine activities and isotope ratios of plutonium and uranium. The method includes dry and wet ashing of air filters and separation of plutonium and uranium from each other and from other nuclides on Eichrom®´s extraction chromatography resins UTEVA and TRU. Interferences in plutonium separation caused by thorium and polonium were investigated and eliminated. A field trial was performed in Kurchatov, Kazakhstan, to test the aerosol sampling and radiochemical methods developed for plutonium and uranium. In the air samples collected, the activity of 239,240Pu varied between 0.67 and 228 Bq/kg of dust, the activity of 238Pu from 0.20 Bq/kg to 16.4 Bq/kg of dust, and the isotopic ratio 238Pu/239,240Pu between 0.011 and 2.4. Concentrations of Pu isotopes in the air were 13.4-2490 nBq/m3 for 239,240Pu and 7.2-960 nBq/m3 for 238Pu. Radioactivity of 239,240Pu in air filters from Astana varied between 0.075 and 0.72 Bq per kg of dust and that of 238Pu between 0.038 and 0.14 Bq/kg of dust. Activity concentration of 239,240Pu in the air of Astana was 5.0-130 nBq/m3.