BACKGROUND: Mast cells are multifunctional cells containing various mediators such as cytokines, proteases, and histamine. They are found in the human heart and have been implicated in ventricular hypertrophy and heart failure. The transplanted bone marrow cell (BMC) at ischemia area of infarct heart can be affected under pathologic environment. We have elucidated the effect of mast cells on BMC. METHODS AND RESULTS: Cultured BMC from Sprague-Dawley rats and neonatal cardiomyocyte were incubated with mast cell granules (MCGs) for 24 hours. The highest concentration of diluted MCGs caused the death of approximately 50% of cardiomyocytes and 70% of BMC. This cell death was proved to be apoptosis, as quantified by TUNEL stain and PI-Anexin analysis. Interestingly, in BMC, MCG-mediated cytotoxicity was induced in low concentration of diluted MCGs, but in CMC. The effects of MCG on the production of stem cell factor (SCF), interleukin-6 (IL-6), TNF-alpha, TGF-beta, and monocyte chemoattractant protein (MCP)-1 in BMC were examined using RT-PCR method. Treatment of BMC with MCG increased SCF and IL-6 production (p value), but not TNF-alpha, TGF-beta, and MCP production. CONCLUSIONS: Our results suggest that MCG treatment induced apoptosis of BMC at low concentration and changed IL-6 and SCF expression. For the transplanted cell survival, the micro-environment of ischemic heart induced by inflammatory cells, especially mast cell, should be considered.