The beta-type transforming growth factor. Mediators of cell regulation in the lung. An increased interest in the role of growth factors in the regulation of processes concerning normal and pathologic lung physiology has spurred a flurry of research in this area. Peptide growth factors are known to control not only cell proliferation but other events such as differentiation, chemotaxis, and matrix deposition as well. The transforming growth factor beta (TGF beta) family of regulatory peptides serves as a prime example to illustrate the multiplicity of effects elicited by peptide growth factors in various lung-derived cell types. At present, the TGF beta family consists of at least 17 proteins and, based on sequence analysis, they can be divided into two groups: a cluster that shows very high sequence similarity to TGF beta 1, the closely related group, and a cluster that shows weaker sequence similarity to TGF beta 1, the distantly related group. The purpose of this brief review is to summarize the salient features of TGF beta structure and regulatory abilities of the closely related group. In addition, we will outline the evidence suggesting a role for TGF beta in normal lung development and physiology. Emphasis will be placed on studies with the closely related members TGF beta 1 and TGF beta 2 because, until recently, purified protein was available only for these two proteins. 