An experimental comparison has been made in a hollow fuel element in PLUTO of a number of possible fast-neutron flux monitors with the object of providing such a flux monitor for irradiations in very high flux materials-testing reactors. If 107 mb is adopted as the reference fast-neutron activation cross section of Ni58, the fast-neutron activation cross sections for the reactions Fe54 (n,p) Mn54 and Ti46 (n,p) Sc46 are found to be 73 mb and 8 mb respectively. It is concluded from this experiment that the Fe54 (n,p) Mn54 reaction using iron enriched to 95% Fe54 will be an adequate long-half-life fast-neutron flux monitor for irradiation in the high-flux facilities such as those likely to be used in, for example, BR-2.