The evidence for mast cell heterogeneity in vivo has grown in the last several years from the initial morphologic and histochemical observations to biochemical and functional criteria. In several species, there appear to be at least two distinct classes of tissue mast cells that differ in their content of intragranular amines, proteoglycans, and proteases and in their metabolism of arachidonic acid after cell activation. With the recent discovery of differentiation/growth factors for mast cells in vitro, it has been shown that a discrete subpopulation of cells resembling one of the two in vivo classes can be derived in culture. The diversity of mast cell mediators synthesized by these various populations and their potential functions in vivo imply that mast cells may be involved not only in allergic responses but also in other inflammatory processes, including tissue repair and growth.