[en] A rapid method has been developed for the estimation of trace amounts of cobalt in biological materials by thermal neutron activation and substoichiometric extraction with 1-phenyl-3-methyl-4-benzoyl pyrazol-5-one (PMBP) in isoamyl alcohol. Cobalt was separated from the active matrix as potassium cobaltinitrite, dissolved in concentrated HCl and finally extracted with PMBP in isoamyl alcohol from a sodium acetate-acetic acid buffer medium of pH 5.5. As low as 0.11 μg of cobalt can be estimated with an accuracy of 10%. The average value of four estimation of cobalt in synthetic mixtures containing 4.40 μg of cobalt is 4.39 μg, which varied between 4.14 μg and 4.65 μg. Two samples and one standard can be analysed within one hour after receiving the irradiated material in the laboratory. (author)