The mechanisms responsible for mesangial cell proliferation in proliferative glomerulonephritis are only partially understood. This article reports the results of an immunohistochemical study showing high expression of the chemokine receptor CXCR3 by mesangial cells of patients with IgA nephropathy, membranoproliferative glomerulonephritis, or rapidly progressive glomerulonephritis. CXCR3 was also detectable by flow cytometry in cultured human mesangial cells, in which it appeared to be functionally active, as determined by the ability of its ligand, the (interferon-gamma)-inducible protein of 10 kD (IP-10) to induce intracellular Ca2+ influx. Both IP-10 and the monokine induced by interferon-gamma (Mig) were also effective in inducing proliferation of human mesangial cells. These data suggest that in patients with proliferative glomerulonephritis, the chemokines IP-10 and/or Mig not only may act as chemoattractants for infiltrating mononuclear cells in the inflamed tissue, but also may directly induce the proliferation of mesangial cells.