AbstractA method for the long‐term culture of rabbit newborn bone marrow has been developed. It is characterized by the rapid appearance of an adherent, adipocyte‐containing stromal layer, proliferation of mature myeloid cells, and the formation of numerous, large multinucleate giant cells. By the combined use of morphological, immunological, and functional criteria these giant cells have been characterized as macrophage polykaryons and not osteoclastic giant cells. We conclude that long‐term bone marrow culture in the rabbit favors the proliferation, maturation, and fusion of macrophage, but not osteoclast, precursors — new experimental models will have to be developed to enable the developmental biology of osteoclasts to be studied in the