Integral measurements of 237Np capture and fission rates have been carried out relative to the 239Pu fission rate in two GCFR lattices with different neutron spectra. The results have been compared with calculated values based, respectively, on ENDF/B-IV and FGL5 cross-sections. Both data sets predicted the measured fission ratios correctly within the experimental errors of ±2% but, in the case of the ratio of 237Np capture to 239Pu fission, the calculated ratios differed from the measured values by about 10% for both sets. © 1979.