The antiferromagnetism of a single crystal is considered, basing on the Van Vleck model, modified by introducing an anisotropy energy of one spin, arising mainly from the crystalline field of the surrounding anions. On this model are derived the anisotropy of the susceptibility above the Curie point, the temperature variation of the anisotropy constant below the Curie point and the field dependency of the susceptibility under a comparatively strong magnetic field.