Latent transforming growth factor-beta 1 binding protein (LTBP) is a constituent of the latent high molecular weight complex of TGF-beta 1 in human platelets. In the present communication, we show that LTBP binds Ca2+ in its free form as well as in the latent TGF-beta 1 complex. The binding of Ca2+ induces a structural change which protects the molecule against proteolysis and changes its elution position when analyzed by anion exchange chromatography. The in vitro activation of TGF-beta 1 is not influenced by the presence of Ca2+. The possible significance of Ca2+ binding of LTBP is discussed.