The authors have endeavoured to determine the place occupied by mast cells in bone tissue from the mode of action on the bone of their secretion products (metachromatic granules, heparin, prostaglandins), from their proliferation in reaction to some bone diseases (secondary mastocytosis) and from the bone lesions associated with systemic mastocytosis. Metachromatic granules are capable of chelating calcium, and heparin and prostaglandins stimulate bone resorption. However, the role of mast cells in bone remodeling through these mechanisms has not yet been demonstrated. Similarly, the significance of mastocytosis secondary to osteomalacia, renal osteodystrophy and ordinary osteoporosis remains hypothetical. Primary bone mastocytosis produces diffuse or circumscribed osteogenic, osteolytic or mixed lesions. While the pathogenesis of osteolytic forms can readily be explained by the actions of heparin and prostaglandins, that of the osteogenic forms is still mysterious. Finally, the histological profile, and even the nosological profile, of mast cell osteoporosis, a diffuse condition recently individualized, remain undetermined.