The complex magnetic permeability of Nb‐44 at.% Ti has been measured using a modified Hartshorn ac bridge and a lock‐in amplifier. The out‐of‐phase signal can be directly related to the energy losses, and critical current can be calculated using the Bean model. Samples were irradiated in a fission neutron spectrum to a dose of 3.2×1018neutron/cm2. All measurements were takenin situat approximately 4.5 °K in an external magnetic field continuously swept to 6.7 kOe while applying ac magnetic fields at 88 Hz with amplitudes between 8 and 500 Oe. Irradiations and measurements were performed on an annealed and a cold‐worked sample of the alloy. For the annealed sample, no changes were observed inVLand thus inJcafter 8.2×1017neutron/cm2and after an anneal to 320 °K. A decrease of approximately 8% was observed inJcfor the cold‐worked sample after 3.2×1018neutron/cm2. One‐half of this decrease was recovered by 270 °K.