The reactivity of myocytes has been ultrastructurally investigated. Pieces of aorta thoracic from mice were organ-cultured on the chorioallantoic membrane of chicken eggs incubated for 14 days. The outstanding finding were individual myocytes of the media – probably in a particular cytoenergetic stage – undergoing necrosis. They were enclosed from almost unaffected myocytes. Transferred to in vivo conditions, these findings may serve as a model for a participation of myocytes in the development of disseminated multifocal arteriosclerotic plaques. The reactivity of the myocytes as to their ability of proteoglycan and lipid synthesis was opposed to Benditt’s hypothesis of the genesis of an arteriosclerotic plaque caused by a monoclonal cell line originating from a mutated myocyte of the artery wall near the site of the plaque. The hypothesis of Kaunitz has been discussed, according to which the high serum cholesterin level with severe arteriosclerosis may be rather a consequence of cholesterin synthesis by the ‘reparative’ tissue of the arteriosclerotic lesion than its reason.