['At present, it is highly controversial whether pure mast cells can serve as antigen presenting cells, and it is not known whether the capacity of antigen presenting function is temporally restricted to a particular subset of differentiated mast cells. Evidence is presented for a novel surface FcepsilonRIhi , MHC II +, and c-kit + pure mast cell subset, temporally restricted as antigen-presenting cells in the immune axis of T-cell activation.', 'Bone marrow-derived mast cells (BMMC) cultured in the presence of IL-3 for three weeks are pure mast cells based on surface expression of lineage-specific marker, c-kit and FcepsilonRI. Herein we present the first demonstration that approximately 98.7% c-kit + and FcepsilonRI expressing BMMC, further depleted of any contaminated professional antigen-presenting cells, are still fully capable of presenting antigens, i.e., OVA protein, OVA peptide, and IgE-TNP-OVA, to OVA peptide-specific T-cell hybridomas. Notably, IgE-dependent antigen presentation is more efficient compared to that resulting from direct antigen uptake. Importantly, we present the novel finding that only surface FcepsilonRIhi mast cells, also expressing surface MHC II exhibited antigen-presenting function. In contrast, surface FcepsilonRIlo mast cells without expressing surface MHC II were not capable of antigen presentation. Interestingly, the antigen-presenting function of BMMC was irrevocably lost during the third and fourth week in IL-3 or SCF containing cultures.'] Is the antigen presentation function of bone marrow-derived mast cells spatiotemporally restricted to a subset expressing high levels of cell surface FcepsilonRI and MHC II?