In organs of rheumatoid inflammation avascular tissue is nourished by perfusion either directly from the blood stream or by neighbouring blood vessels (borderline-tissue). An open question remains whether this borderline-tissue promotes only the known antigen persistence, or also the first deposition of the pathogenetically important microbial antigen, and subsequently a deposition of immune complexes. The three markers (carbon, latex, and living bacteria) used in this study led to nearly the same pattern of deposition in two groups of organ systems: Organs with clearance function belonging to the mononuclear phagocyte system (MPS); organs with borderline-tissues: joint, kidney, artery, heart valve, and eye. This deposition at the borderline can be observed best in areas of the joint where highly vascularized tissues are adjacent to avascular tissues nourished via perfusion: perichondrium, peritendineum, periosteum, and endo- and perimysium. These localisations of particle deposition correspond exactly with the intraarticular predilection sites of chronic rheumatoid inflammation in man and animals. Our results indicate an easier settlement of corpuscular material in these localisations, which at the same time are unable to eliminate such substances, unlike the organs of the MPS. Our studies seem to support the theory of pathogenetic importance of the borderline-tissues as a localizing factor in the perpetuating rheumatoid inflammation.