The authors measured at 20K the magnetoresistance curves of two high-purity polycrystalline iron samples in small longitudinal magnetic fields (H/sub ///<1000 G): (i) during a 2.5 MeV electron irradiation, (ii) during the recovery of the damage. They show that the resistivity anisotropy of their samples ( delta rho = rho /sub ///- rho perpendicular to ) is closely connected to their residual resistivity rho 0. During the irradiation (increase of rho 0 due to the increase of scattering centres) the authors go gradually from a region of negative delta rho (the 'normal' magnetoresistance is predominant) to a region of positive delta rho in which the spontaneous anisotropy dominates. The authors can deduce from their measurements the value delta rho / rho /sub ///=0.3% of the spontaneous anisotropy relative to the diffusion by Frenkel pairs.