The neutron emission rate of a Ra-α-Be source of the National Research Council has been measured by moderating the neutrons in manganese sulphate solution surrounding the source. Some of the thermal neutrons captured in the bath produce manganese-56, and the neutron emission rate is found from an absolute determination of the resulting activity. A number of corrections are discussed in detail. The value of (3.22 ± 0.05) × 106 neutrons per second found for the neutron emission rate shows satisfactory agreement with the results of our intercomparison with the primary neutron standard of the National Bureau of Standards, and allows a tie-in with a series of international comparisons of primary neutron sources.