The ultrastructural distribution of Treponema pallidum and the host inflammatory response during experimental testicular infection of rabbits have been examined. During the inductive phase of experimental orchitis both organisms and inflammatory cells, primarily lymphocytes, are coated by amorphous material not seen in other cellular inflammatory reactions. Phagocytosis of organisms by macrophages occurs during the reactive phase. Phagocytic vesicles contain T. pallidum and are frequently lined by amorphous material. T. pallidum do not appear structurally abnormal prior to phagocytosis; destruction of T. pallidum, manifested by swelling and lysis of the organisms, occurs within phagocytic vacuoles. We conclude that removal of organisms during the reactive stage of experimental syphilis is accomplished by phagocytosis and digestion of organisms by macrophages, resulting from a delayed hypersensitivity reaction initiated by specifically sensitized T cells. The presence of amorphous "ground substance" material does not block the inflammatory response and inhibits neither phagocytosis nor digestion by macrophages.