Abstract: By using a combination of non-isotopic carrier distillation and CuS-coprecipitation under pressure, trace amounts of osmium in the metallic grains of Allende and Leoville are separated from Fe-Ni matrix as a radioactive pure fraction. Activation analysis is used to determine the concentration of osmium down to 10-10g and other refractory siderophile elements and the isotopic ratios Os-184/Os-190, the determination of which seems very difficult by ordinary mass spectrometry.