Multifilamentary composite wires of Nb Ti and Nb3Sn have been irradiated at 60 ± 5°C with fast neutrons to fluences of 6 × 1019 n/cm2. Measurements of the superconducting critical current, Ic , as a function of transverse field show that the NbTi wires are only moderately affected by neutron irradiation. At a fluence of 6 × 1019 n/cm2, Ic (40 kG) is 82% of the unirradiated value. The Nb3Sn composites undergo a catastrophic reduction in Ic with an apparent threshold at a fluence of 2 to 3 × 1018 n/cm2. Between 2 to 3 × 1018 and 1.1 × 1019 n/cm2, Ic (40 kG) has been reduced to 4% of the unirradiated value. At a fluence of 6×1019 n/cm2, the upper critical field of Nb3Sn has been reduced from 240 kG to 9 ± 0.5 kG. The corresponding Tc has been decreased from 16.4 to 6°K. Annealing of Nb3Sn samples irradiated to 1.1 × 1019 n/cm2 produces only 19% recovery in Ic after h at 400°C.