The problem of interpreting Mossbauer spectra of antiferromagnetic powders subject to applied magnetic fields is considered. A mean-field model for systems with uniaxial anisotropy is used to fit simultaneously spectra taken with applied fields of different strengths. The model predicts a variety of asymmetric broad line profiles depending on the strengths of the exchange and anisotropy terms. The method is applied to the 4.2 K spectra of goethite and haematite subject to longitudinal applied fields of 0, 3, 6 and 9 T. Favourable agreement is obtained with the results from single crystals, showing that the method can be reliably used for a variety of antiferromagnetic materials.