Mast cells with the morphological and biochemical properties of mucosal mast cells (MMC) appear and proliferate to form the predominant cell type in rat bone marrow cultures stimulated with factors from antigen or mitogen-activated lymphocytes. This thesis has been divided into two sections; in the first section, a detailed examination was undertaken of the culture conditions required, including factor dependency, to cause the selective emergence and proliferation of MMC from normal bone marrow cultures. In the second section, the author has attempted to characterise the cells growing in culture using histological, morphological and immunohistochemical techniques.