The concerted action of TGF-β and PDGF on a diploid human embryonic lung fibroblast cell strain (Flow 2002) grown in an homologous environment is investigated here. In sparse cultures, TGF-β stimulates DNA synthesis over a broad concentration range (0.1-10 ng/ml). Furthermore, it acts in synergism with PDGF, a phenomenon which persists also during in vitro aging of the cells. Preincubation of TGF-β with the fibroblasts up to 12 hours reduces the subsequent PDGF binding to the cells, while prolonged preincubation restores PDGF binding to control levels. Finally, in cultures of higher cell densities, TGF-β ceases to stimulate DNA synthesis, whereas PDGF continues even at cell confluency, retains its stimulatory activity suggesting different roles for the two growth factors during the wound healing process.