Evidence has accumulated suggesting that the various isoforms of β-type transforming growth factors (TGF-βs) regulate important functions in the lung; however, the cellular source of these proteins is not well defined. Northern blot analysis of murine lung tissue demonstrates that mRNA transcripts for all three TGF-β isoforms are found from birth through adulthood. Although the level of expression for each TGF-β is variable during the first 2 wk post partum, all three isoforms are equal in the adult lung. Using in situ hybridization and immunohistochemical analysis, we have localized both mRNA and protein expression for all three isoforms of TGF-β in the adult murine lung. At low magnification, immunohistochemical localization of TGF-β proteins appears coincident in their pattern of expression with TGF-β mRNAs in the large proximal conducting airways of the lung. However, on closer analysis, protein expression of all three TGF-β isoforms is confined to the bronchiolar epithelium, while TGF-β mRNA transcripts for each of the TGF-β genes are found in smooth muscle cells and connective tissue fibroblasts lying subjacent to the epithelium. Although the levels of both TGF-β mRNA and protein expression are high in the proximal bronchiolar tree, their signal intensities completely disappear as the terminal bronchioles progress to respiratory bronchioles. Additionally, in the lung vasculature, there is very high expression of all three TGF-β mRNA transcripts in the smooth muscle cells of the large vessels. TGF-β₂ and TGF-β₃ but not TGF-β₁ proteins are expressed in these same smooth muscle cells. The temporal and differential cellular localization of mRNA synthesis and protein expression in lung tissue provides evidence for a regulatory role for each of these proteins in cellular processes of the lung. Comparison of TGF-β mRNA and protein localization suggests that both paracrine and autocrine mechanisms of TGF-β activity are present in the lung.