The authors describe a method for the radiochemical assay of cobalt based on the formation of cobalt 60m after activation with thermal neutrons; the conditions of irradiation, the method for determination of loss during separation, also the influence of manganese 56 and of iron on this estimation, are given in detail. Following the procedure indicated it is possible to estimate as little as 0,07μg of cobalt with an accuracy of ± 7%. For a single analysis the time required is 46,5 min., for ten analyses approximately 2 h.