The preparation of a polonium–beryllium sandwich containing 2.1 c. of polonium is described. The source emits 2 × 106neutrons per second and its γ radiation is approximately equivalent to that of 0.2 mgm. radium in equilibrium with its decay products. Polar diagrams for the angular distribution of the fast neutrons are given. Emission data for two similar neutron sources, containing 3.2 and 3.5 c. polonium respectively, are given.