The authors submit a method for the rapid separation of cobalt by toluene extraction of its β-nitroso-α-naphthol complex, with estimation of the losses involved. Iron, the chief element causing disturbance in assay by gamma spectrometry (⁶⁰Co) after activation, gives a complex with sodium fluoride-acetate-citrate in a medium at pH 5–6 and remains quantitatively in the aqueous phase. In this way it has been possible to separate 0,1 μg of cobalt in presence of 100 mg of iron. The estimation of losses is carried out by means of a radioactive tracer (⁵⁸Co), isotope of the element being assayed (cobalt 60 or 60 m). The time necessary for separation and estimation is about 20 min and would thus permit a cobalt assay based on that of the cobalt 60 m (periode 10,5 min) wich is formed by activation. A chemical method for the control of losses has also been studied.