The replication of fibroblasts is thought to be controlled by exogenous growth factors mainly secreted by macrophages and epithelial cells. However, under standard culture conditions, lung fibroblasts are able to produce several growth factors, suggesting an autocrine pathway of proliferation. In this work, we examined the expression of platelet-derived growth factor (PDGF-A) and PDGF-B-messenger RNA (mRNAs) by fibroblasts derived from four human adult normal lungs and from two fibrotic lungs. Northern blot analysis showed that both normal and idiopathic pulmonary fibrosis (IPF)-derived fibroblasts expressed a 2.8 PDGF-B/c-sis mRNA. This transcript was also observed as a minor form in human osteosarcoma cell line, used as control, which predominantly expressed a 4.0-kb PDGF-B mRNA. In two fibroblast cell lines, one fibrotic and one normal, the 4.0-kb transcript was also observed but was always weaker than the 2.8-kb mRNA. PDGF-A mRNA was not detected. By immunofluorescence, lung fibroblasts exhibited intracytoplasmic PDGF-like protein. Likewise, conditioned media from normal and IPF lung fibroblasts stimulated 3H-thymidine incorporation in BALB/c-3T3 cells that was significantly inhibited by anti-PDGF antibody. These results show that in vitro, some human lung fibroblasts express PDGF-B/c-sis mRNA, mainly an alternate 2.8-kb transcript, and produce PDGF-like protein.