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Several carriers mediate ionic fluxes across the plasma membrane in a variety of mammalian cell types. Intracellular proton concentration is regulated by virtue of the operation of at least two distinct systems: a stilbene-sensitive, Na+- dependent HCO3-/Cl- exchange system, and an amiloride-sensitive Na+/H+ antiporter. The contribution of these two transporters to the modulation of intracellular pH in response to either extracellular pH variations or cell stimulation by growth factors and tumor promoters has been studied in several cell lines, including fibroblast mutants lacking Na+/H+ antiport activity. The attainment of a permissive intracellular pH value is critical to the development of the mitogenic response elicited by growth factors. Kinetic studies have revealed particular features of the Na+/H+ antiporter that explain its function in the early sequence of biochemical events leading to DNA replication. The detailed investigation of the mechanisms by which protons and other ions might regulate cell proliferation has important implications for the understanding of the role of pH microenvironment in carcinogenesis, tumor development and chemotherapy.