AbstractWe report here that transforming growth factor-β1 (TGF-β1) inhibits platelet-derived growth factor (PDGF)-induced DNA synthesis in normal human fibroblasts in a cell density-dependent manner; no inhibition was seen in sparse cultures, approximately 50% inhibition in confluent cell cultures, and an almost total inhibition in dense cultures. The PDGF-inducible genesc-mycandc-foswere induced also by TGF-β1. Simultaneous addition of TGF-PI and PDGF resulted in sustained, rather than transient, expression ofc-fosmRNA; c-fosmRNA was detected as long as 24 hr after addition of PDGF and TFG-β1. TGF-β1 also induced mRNA for the A chain, but not the B chain, of PDGF. Conversely, PDGF induced TGF-β1 mRNA in sparse but not in dense cultures. These data indicate the existence of a complex interdependent regulation of PDGF and TGF-βmRNA expression which is influenced by the cell density.