Transforming growth factor beta (TGF-β) binds specifically and with high affinity to several different cell surface proteins. Low M(r) proteins of 50,000 and 80,000 have been termed type I and type II receptors. Intermediate sized binding components of 115,000-140,000 M(r) and a high binding components of approximately 250,000 M(r) in subunit size have been termed type III receptors. The high M(r) component is a proteoglycan containing the glycosaminoglycan chains of heparan sulfate and chondroitin sulfate and the intermediate sized components are its core proteins. Although almost all cells have TGF-β receptors, binding of TGF-β to the type III binding components is restricted to cells of fibroblastic, osteoblastic and chondroblastic origin. The physiological relevance of each individual binding class is unclear. However, recent data indicate that the type III protein does not transmit signals to inhibit cell proliferation, induce protein synthesis, or promote cytomorphological change and that these activities may be mediated through the type I receptor. The mechanism of signal transduction remains unknown, but it does not appear to be associated with tyrosine phosphorylation or phosphorylation of the 40s ribosomal protein S6.