Immunolocalisation and histochemical techniques were used to examine mineralised bone deposits within late stage atherosclerotic plaques of human carotid arteries. These specimens showed characteristic features of osteogenesis. Large calcifications were often observed in close association with or integrated within mineralised bone. Smooth muscle cells (alpha-actin positive) were often located around osteoid-like matrix, together with focal accumulations of macrophages (CD68 and HAM56 positive). Local accumulations of mast cells (tryptase positive) were consistently observed in close association with the bone. Multinucleated giant cells in close apposition with mineralised bone demonstrated typical osteoclastic morphology, and were positively stained for acid phosphatase and the macrophage marker CD68. Thus, all the normal features of bone formation and resorption were observed in this microcosm of osteogenesis within atherosclerotic plaque; the term 'osteosome' seems appropriate for the structure. These osteosomes have numerous advantages for experimental studies of the various osteogenic factors responsible for bone metabolism, especially following short-term tissue culture. This ex vivo technique was used to demonstrate the distribution and the multiple cellular sources of bone morphogenetic protein 6.