Mast cells (MCs) play an important role in both inflammatory and immunosuppressive responses [1]. The importance of MCs in maintaining peripheral tolerance was discovered in a FoxP3(+) regulatory T-cell (Treg)-mediated skin transplant model [2]. MCs can directly mediate tolerance by releasing anti-inflammatory mediators (reviewed in ref. 3) or by interacting with other immune cells in the graft. Here we will present protocols used to study the role of MCs in peripheral tolerance with the emphasis on how MCs can regulate T-cell functionality. First we will introduce the skin transplant model followed by reconstitution of mast cell-deficient mice (B6.Cg-Kit (W-sh) ). This includes the preparation of MCs from the bone marrow. Finally the methods used to study the influence of MCs on T-cell responses and Treg functionality will be presented by modulating the balance between tolerance and inflammation.