Both analytical and numerical models are developed to investigate the effects on alternating field (AF) stability of both microcoercivity associated with pinned walls and magnetic screening by soft walls in multidomain (MD) grains. It is shown that due to screening, AF stabilities of remanence in MD grains are usually significantly higher than their microcoercivities, the only exception being for two-domain grains, in which there is no screening. One important implication is that hard ''pseudo-single-domain'' remanence observed in grains larger than single-domain size but with AF stabilities higher than the expected maximum microcoercivity associated with domain wall pinning may be simply due to the screening by soft walls in individual grains. Moreover, we predict that MD grains containing a small number of defects do not necessarily show a low AF stability, for the reduction in the AF stability by a small microcoercivity in this case may be outweighed by a large screening due to a relatively large number of soft walls available in the grains. This prediction is supported by the experimental evidence that two types of magnetites (crushed and hydrothermally grown) with very different defect densities have similar values of median destructive field. ¿ American Geophysical Union 1993