Some domain imaging studies have been used as evidence for transdomain transitions and metastable single-domain states in particles larger than theoretical single-domain (SD) size. Domain imaging has certain limitations since observations are made of a two-dimensional surface, which is generally polished and may introduce stress effects, and usually a relatively small number of particles are analyzed. The present paper shows how field-impressed anisotropy of magnetic susceptibility provides independent quantitative support in three dimensions for transdomain transitions and metastable SD states. In particular, changes from positive to negative impressed anisotropy (prolate to oblate impressed ellipsoids) in a direct field (DF) of increasing strength are consistent with some small multidomain (MD) particles undergoing transitions to metastable SD states. The results suggest that DF treatment can quantify the particle size limits of metastable SD behavior. In contrast, an alternating field (AF) should not produce a metastable SD state in particles greater than theoretical SD size and experimentally produces positive impressed anisotropy in MD particles and negative values only in intrinsic SD particles. Consequently, AF treatment can be used to determine the particle size limits of theoretical classical (not metastable) SD behavior. Consistent results were obtained for particle size fractions of magnetite, titanomagnetite, pyrrhotite, and hematite. The results may also help to explain recent observations concerning the low-field variation of AC susceptibility of various minerals. Field-impressed anisotropy is a quantitative, rapid, three-dimensional technique, requiring no extra sample preparation (thus not introducing stress effects), and representative of a statistically large number of particles. Copyright 2006 by the American Geophysical Union.