A high-sensitivity scintillation β spectrometer was designed to measure the 90Sr contamination of environmental samples against a background of 40K and 137Cs radionuclides. The plastic scintillator detector with a developed surface also acts as the sample container. The volume of the sample being analyzed is 1000 cm3. The minimum detectable specific activity of 90Sr is 3.5 Bq/kg for solid granular samples and 1 Bq/L for water. (The measurement time for both samples and background is 2 h.)