The macromolecular matrix present in the composite shell of the blue mussel, Mytilus edulis, accounts for less than 1% of the shell by weight but is theorized to play a significant role in controlling the growth, morphology, and orientation of the CaCO3 that makes up the shell. The presence of several proteins in this matrix, only some of which have affinity for calcium, suggests a hierarchical structural model for the shell. Proteins were isolated under denaturing, reducing conditions and separated by centrifugation, gel electrophoresis, and high performance liquid chromatography. The major matrix proteins, both soluble and insoluble, were evaluated for amino acid composition, calcium binding, and glycosylation. Some N-terminal sequence data was collected. Non-proteinaceous components of the matrix were also analyzed. Comparison of the mussel shell matrix with the protein matrix of other molluscan systems suggests that this complexity is not unique to the mussel and may provide a key to the understanding of more generic biomineralization processes necessary for such applications as biomimetic ceramics.