The application of alternating (AF) of direct (DF) magnetic fields to two weakly anisotropic rock specimens which remain static, significantly alters their measured low-field anisotropy of magnetic susceptibility (AMS) by superimposing an anisotropy which has the form of an ellipsoid of revolution with its unique axis aligned along the field axis. Previous AF history is also shown to influence the acquistion of isothermal remanent magnitization (IRM). These effects can be quite substantial and thus it is essential that rocks should not be used for low-field AMS analysis or for IRM anisotropy studies if they have previously been subjected to static AF demagnetization. AMS studies should also not be carried out on rocks which carry a significant remanance (e.g., an IRM acquired in ≥5 mT). The field-impressed effect were also strongly dependent upon whether the ferrimagnetic particles in the rocks were predominantly multidomain (MD) or uniaxial single-domain (SD). The results were consistent with our recent work on synthetic specimens (containing magnetite, titanomagnetite or maghemite) and add support to our suggestion that these effects could provide a rapid, nondestructive, means of determining the predominant domain state (MD or uniaxial SD) of ferrimagnetic particles in rocks. ¿American Geophysical Union 1990