The relationships between the magnetoelectric (ME) susceptibilities in polycrystalline powders and single crystals are derived for antiferromagnetic magnetic crystal classes possessing orthorhombic or higher symmetry. The powder susceptibilities induced by both parallel and perpendicular ME annealing are calculated in terms of the single‐crystal ME susceptibilities. The results indicate that: (a) Measurements of the ME effect in a powder specimen, together with a knowledge of its crystallographic point group in the ordered state, are sufficient to determine its magnetic crystal class. (b) In many cases, the elements of the single‐crystal ME susceptibility tensor can be determined, at least in principle, from the powder measurements. The application of these results to potentially ME materials is discussed.