In this study the development of sterile thrombic vegetations on the aorta resulting from catheterization and the effect of subsequent infection with Staphylococcus aureus were examined by light and electron microscopy. Thrombi of various sizes, comprising fibrin, platelets and a few leucocytes and erythrocytes, develop on the damaged surface of the aorta with minimal changes in the underlying aortic wall. After intravenous inoculation of Staph. aureus most vegetations become infected, as shown by the presence of bacterial colonies, and the underlying aortic wall is markedly inflamed. The inflammatory cells invade the wall from the base of the aorta and cause swelling plus disruption of the elastic laminae with ulceration of the luminal surface in some cases. This structural damage appears to be a direct result of the bacterial infection of the lesions on the luminal surface.