The absolute neutron flux produced in a Van de Graaff accelerator via the 7Li (p, n).Be-reaction was measured with the help of a circulated manganese bath. An elaborate process was developed for the absolute determination of the efficiency of the gamma detector adapting the circulating system. The absolute emission rate of an Am-Be neutron source was also determined with the bath method, which agrees well with the calculated value according to the given decay rate of 241Am.