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The anisotropy field distributions in powders of BaFe12O19 were measured by the remanent-torque-curve method. Milling of a powder increased the percentage of particles with low anisotropy fields and reduced the average anisotropy field HA. Annealing the powder decreased the percentage of such particles and raised HA. The intrinsic coercive force MHC was a single-valued function of HA, for samples derived from a common starting powder. The points for samples derived from different, independently prepared powders did not all lie on the same curve, but it is suggested that each independently prepared powder has a curve of its own associated with it. These results are explained on the basis of lattice dislocations, introduced by milling and healed by annealing. These dislocations reduce the magnetocrystalline anisotropy, and a mechanism for this effect is briefly described.