A method is given for calculating the minor and full hysteresis loops of a system of uniaxial single-domain particles with a common direction of easiest magnetization. A probabilistic approach is used in formulating the problem by means of two differential equations which are solved for the simple case of an infinite particle chain. The method is based on a model in which each particle is characterized by a rectangular hysteresis loop with a specific saturation field H s , and the loops are influenced by the (homogeneous) external magnetic field H as well by the magnetization-dependent interaction of the particles among themselves.