NbTi and Nb3Sn multifilamentary composites have been irradiated with fast-neutrons at 60 ± 5°C to fluences of 1.2×1020n/cm2(E > 1 MeV). The NbTi samples show only a moderate reduction of Icas a function of neutron fluence in an applied field of 40 kG. Reductions in Icwere observed for fluences greater than 3 × 1017n/cm2and saturate at 18% for fluences greater than 3-4 × 1019n/cm2. The Nb3Sn composites showed large neutron radiation induced changes in Tc, Icand Hc2. Reductions in Tcwere observed for fluences greater than 7 × 1017n/cm2. No measurable changes in Ic(40 kG) were observed below 1018n/cm2. Between 2 and 3×1018n/cm2, however, there is an apparent threshold where a very rapid reduction in Ic(40 kG) is initiated. At the threshold the decrease in Tcis 13%. Between the threshold and 1.1 × 1019n/cm2, I2(40 kG) has been reduced to 4% of the unirradiated value. These changes in superconducting properties in NbTi and Nb3Sn are analyzed in terms of the radiation induced defects. The impact of the response to irradiation of both materials on their applications in fusion reactor magnets is discussed.