Thin sections of pathologically changed skin lesions of patients suffering from purpura pigmentosa progressiva, a disorder which seems to be mediated by delayed type of allergy, were electron microscopically investigated. In 5 of the 7 cases we found cell infiltrates of the dermis consisting of many lymphocytes frequently in apposition to macrophages (histiocytes). The cell interactions observed occurred either by microvilli-like cytoplasmic protuberances or by local areas of the plasma membrane apposition. No submicroscopical alterations of the cytoplasm were found in the interacting cells although numerous macrophages show abundant microfilaments. The findings observed are interpreted as the ultrastructural substrate of cell cooperation for presenting and transfering antigens from macrophages to T lymphocytes. However, the question concerning the mechanism of the transfer remains open.