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Platelet-derived growth factor (PDGF) and its receptor system regulate mesenchymal cell proliferation. We recently reported that emission-source fly-ash particles and asbestos fibers induce the PDGF α-receptor through a macrophage-dependent pathway, and upregulation of this receptor greatly enhances the mitogenic response of lung myofibroblasts to PDGF (Lindroos and colleagues, Am. J. Respir. Cell Mol. Biol. 1997;16:283–292). In the present study we investigated the effect of particulate matter ⩽ 10 μm in size (PM₁₀) from the southern, central, and northern regions of Mexico City on PDGF receptor induction and compared these urban, ambient particles with Mt. St. Helen's volcanic ash particles as a negative control. All Mexico City PM₁₀ samples, but not volcanic ash, stimulated rat alveolar macrophages to secrete a soluble, upregulatory factor(s) for the PDGF α-receptor on early passage rat lung …