Abstract Five cases of acute myelogenous leukemia with massive mast cell infiltration in bone marrow and leukemic tumors of the soft tissue were examined to elucidate the relationship between leukemic cells and mast cells. Leukemic cells in each case showed no basophil/mast cell features and differentiated to neutrophil-like cells in liquid culture. Isolated mast cells failed to proliferate with or without growth factors used irrespective of culture conditions. Mononuclear cell fraction obtained from all patients gave rise to relatively numerous mast cells in interphase culture using IL-3 but not in other culture systems. The simultaneous addition of leukemic cell membrane and IL-3 or delayed addition of leukemic cell membrane produced more numerous and mature mast cells in the upper layer of interphase culture using IL-3. However, addition of leukemic cell membrane alone failed to increase the number of mast cells. Similar results were obtain using mononuclear cells obtained from volunteers and cord blood cells. These results indicate that a cell to cell contact or interstitial substance together with IL-3 are essential requirements for growth and maturation of mast cell precursors and that the cell membrane itself in some cases of leukemia has the ability to induce maturation of mast cell precursors.