THE isotope Ba139 is radioactive with a half-life of 85 min. decaying to La139 by β− emission. This activity was first reported by Pool and Cork1. Kalbfell and Cooley2, using absorption methods, have reported a continuous β-spectrum with a maximum energy of 1 MeV. accompanied by γ-radiation of energy 0.6 MeV., while Born and Seelmann-Eggebert3 report a maximum β-energy of 2.3 MeV. also by an absorption method.