A calculation of the thermal neutron self-absorption for cylindrical or spherical neutron sources has been made. The calculations are confirmed by the experimentally-measured difference in manganous sulfate bath activity for bare and cadmium-covered Pu-Be and Am-Be neutron sources. The calculation is done in single interaction approximation and assumes that the incident thermal neutron flux is isotropic. The source material may be fissionable and be covered by up to three cladding materials. A computer program has been written for the numerical calculations.