Response measurements of a Bonner sphere spectrometer (BSS) were performed for high-energy neutrons up to 400 MeV in quasi-monoenergetic neutron fields at Research Center for Nuclear Physics (RCNP), Osaka University. The quasimonoenergetic neutron field was generated by injecting protons on to the Li target with the spectra consisting of a monoenergetic high-energy peak followed by an unwanted continuum tail down to the low energy region. Two-angle differential measurements were performed to obtain the monoenergetic response by subtracting the tail contribution. Discussions were made on data analysis for tail subtraction. While each Bonner sphere had different energy responses, good agreements were seen in the monoenergetic responses between the MCNPX calculation and the measurement with a suitable subtraction. Response calculations were also conducted for the whole spectra obtained by unfolding the measured BSS data and gave the results reproducing well the measured responses. © 2010 IEEE.