Normal lung architecture is related to the presence of mesenchymal cells (fibroblasts and smooth muscle cells), and to the production by these cells of extracellular matrix. The turnover of mesenchymal cells is under a fine regulation due, at least in part, to the local presence of different mediators acting on their cell cycle. Since normal human alveolar macrophages obtained by bronchoalveolar lavage (BAL) spontaneously release platelet-derived growth factor (PDGF), a cytokine with chemotactic and growth activity on mesenchymal cells, we evaluated normal human epithelial lining fluid (ELF) for the presence of PDGF. Active only as a dimer, PDGF is a glycoprotein composed of two chains (A and B) and, thus, can be present in three forms: AA, AB, and BB dimers. Interestingly, normal ELF contains PDGF AA dimers, and to a lesser extent AB dimers, while no significant level of BB dimers is detected. Furthermore, ELF PDGF is biologically active and responsible for a significant part of the chemotactic activity and the "competence" growth activity for mesenchymal cells present in normal ELF. These findings suggest that ELF PDGF has a role in normal lung structure maintenance and tissue repair.