Results of detailed studies on tissue reactions to Cysticercus bovis in the heart of cattle, together with a comparison of findings in animals with spontaneous and experimental infection, and an evaluation of tissue reactions in relation to the location, morphology and morphogenesis of C. bovis provided evidence for the fact that in general, the response of the heart to the presence of C. bovis was an inflammatory reaction characterized by the origin of a pseudoepithelial border and a zone of granulation tissue. Later, when necrotic changes started to affect the cysticercus, the inflammatory reaction started to develop anew. It was accompanied by an exudation and a subsequent resorption. Characteristic features were focal necroses both of the exudate and the inflammatory border followed by a dystrophic calcification, focal necroses and a dystrophic calcification of individual collagenous fibres and groups of these fibres in addition to a regular appearance of necrotic-like foci typical of a reaction to C. bovis. The location of the cysticercus in a certain part of the lymphatic system of the heart and in skeletal muscles was shown to have a considerable effect on the course of the inflammatory reaction. Differences in the development of the inflammatory reaction explained concomitant findings of dead and live cysticerci at the same time after an experimental infection.