Assemblies of glycoproteins from within the mineralized tissues of sea urchins and mollusks both interact in vitro in a similar manner with growing calcite crystals. A protein-rich fraction, a polysaccharide-rich fraction, and a fraction composed of densely glycosylated peptide cores were obtained by chemical and enzymatic treatment of the glycoproteins from sea-urchin spines. Each fraction was partially purified and characterized (amino acid composition, FTIR and NMR spectroscopy). A comparison of the interactions of these fractions with growing calcite crystals in vitro shows that the polysaccharide moieties of these glycoproteins are intimately involved in the interaction with growing calcite crystals on planes approximately parallel to the c crystallographic axis. Presumably the polysaccharides in the mollusk-shell glycoproteins are likewise responsible for the similar interactions of these macromolecules with calcite. We suggest that structured polysaccharide moieties of glycoproteins are important in controlling aspects of crystal growth in vivo as well.