The purpose of this International Standard is to provide the user with a means of verifying the performance of typical liquid-scintillation counting systems. Measures of pe rformance considered in this standard are: - counting system efficiency; - reproducibility of sample and background count rates. This standard does not cover the calculation of sample activity for quenched samples. Accordingly, this standard does not deal with sample preparation, efficiency correlation (quench correction) procedures, or the identification of unknown radionuclides.