The small latent TGF-beta complex often is associated with the latent TGF-beta binding protein (LTBP). Three LTBPs (LTBP-1, -2, and -3) have been isolated to date. Previous studies have shown that LTBP-1 binds the small latent TGF-beta 1 complex through a disulfide bond between an 8-cysteine structural motif of LTBP-1 (TGF-bp repeat) and the propeptide dimer of latent TGF-beta 1 (TGF-beta 1 latency associated peptide). There is indirect evidence that LTBP-2 and LTBP-3 also bind the latent TGF-beta complex, but the nature and location of the binding interaction are unknown. We have used immunoprecipitation, SDS-PAGE, and autoradiography to characterize the association between mouse LTBP-3 and the small latent TGF-beta 1 complex. We report that the second and third TGF-bp repeats of LTBP-3 covalently bind the latent complex, and we show a similar capability for the homologous TGF-bp repeats of mouse LTBP-2. The second TGF-bp repeat of LTBP-3 is unusual in that it has 9 cysteine residues instead of 8, and our results provide the first evidence that a TGF-bp repeat with an odd number of cysteine residues can covalently bind latent TGF-beta 1. Altogether, these results have important implications for TGF-beta biosynthesis and the regulation of TGF-beta activity.