The ultrastructure of intercellular connections between plasma cells and dendritic bone marrow macrophages found in bone marrow aspirations from 15 patients with multiple myeloma is described. Two types of cell-to-cell contact were observed: 1) a juxtaposed type and 2) a mortise-joint type. In both types the zones are characterized by an amorphous electron dense layer adherent to the inner leaflet of the cytoplasmic membrane of the plasma cell, whereas no morphological changes are present in the macrophage. The thickness of the electron dense layer in the plasma cell was found to range from 11 to 33 nm and its extension along the membrane fron 0.2 micronm to 2--3 micronm. The distance from plasma cell surface to the surface of the macrophage was found to range from 11 to 27 nm, but the two cell membranes were always parallel at the contact zone region. At the contact zones small amounts of electron dense material were generally present in the interspace between the cells, and sometimes fine bridges of this material were seen to connect the two cells. The cytoplasm of the plasma cell was sparse in organelles at the contact zone region except for cytoplasmic filaments (7--9 nm in diameter). However, more than 50 per cent of the plasma cells did show a few electron dense, membrane-bound granules close to the contact zones. In some plasma cells a few coated vesicles were also found adjacent to the dense layers of the contact zones.