Measurements have been made of the helium generated by (n,α) reactions in boron that have resulted from several fast-neutron irradiations in the Experimental Breeder Reactor II (EBR-II) and the Coupled Fast Reactivity Measurements Facility (CFRMF). The neutron environments were characterized using multiple-foil dosimetry and reactor physics calculations. Precise helium determinations of boron specimens in milligram size were made using a high-sensitivity-gas mass spectrometer system specifically designed for this purpose. Spectrum-integrated helium -production cross sections have been determined from the measured helium concentration data and the experimentally measured values of total neutron flux. Good consistency has been found between the measured reaction rates of10B and 235 U. In the case of certain other dosimetry foils where cross sections are less accurately known, inconsistencies of up to 30% in predicted reaction rates have been found in EBR-II neutron spectra perturbed by the presence of B4C. This indicates that the 10B(n,α)7reaction is a very useful addition to multiple-foil dosimetry sets, and that its use will help to establish more accurate cross-section data for other foil reactions.