This paper reports the molecular cloning of a novel gene in the mouse that shows structural similarities to the microfibril protein fibrillin and to the latent transforming growth factor-β (TGF-β) binding protein (LTBP), a component of the latent TGF-β complex. The gene was initially isolated during a low stringency polymerase chain reaction screen of a NIH 3T3 cell cDNA library using primers that amplify a human fibrillin-1 epidermal growth factor-like repeat. Three lines of evidence suggest that the mouse gene is a third member of the LTBP gene family, which we designate LTBP-3. First, the deduced polypeptide, which consists of 15 epidermal growth factor-like repeats, 3 TGF binding protein repeats, and 2 proline- and glycine-rich sequences, shows 38.4% identity with LTBP-1 but only 27% identity with fibrillin-1. Second, the gene appears to be co-expressed in developing mouse tissues with TGF-β. Third, immunoprecipitation studies using mouse preosteoblast MC3T3-E1 cells and a specific anti-peptide polyclonal antiserum reveal that the mouse polypeptide forms a complex with the TGF-β1 precursor. Finally, we note that the LTBP-3 gene was recently localized to a distinct genetic locus (Li, X., Yin, W., Perez-Jurado, L., Bonadio, J. and Francke, U. (1995) Mamm. Genome 6, 42-45). Identification of a third binding protein provides further insight into a mechanism by which latent TGF-β complexes can be targeted to connective tissue matrices and cells.