As a first step for probing Ca sites in biomolecules using solid-state Ca-43 NMR spectroscopy, natural abundance Ca-43 NMR data of calcium-containing organic solids is reported. The Ca-43 NMR interaction parameters indicate small electric field gradients and a relatively small chemical shift range for the calcium sites in these bio-related model compounds. A correlation of the Ca-43 isotropic chemical shift with Ca-O distance is found: the Ca-43 shift increases as the mean Ca-O distance decreases, with a strong deshielding being observed for calcium with strong Ca-O bonds (distance < 2.4,angstrom) in both organic and inorganic materials. (c) 2006 Elsevier B.V. All tights reserved.