Extracellular calcium concretions found in the gills of freshwater unionids are largely inorganic, containing calcium and phosphate. The organic fraction of the isolated concretions accounts for 25% of their dry weight as determined by ashing. The organic fraction of isolated concretions was examined using SDS‐polyacrylamide electrophoresis. The concretion fraction is separated into 13–14 protein bands. These bands can be further differentiated into EDTA soluble proteins and insoluble core proteins. One of the soluble proteins has a molecular weight of approximately 17,000 daltons (Da) and comigrates with authentic vertebrate calmodulin. The 17,000‐Da concretion protein cross‐reacts with sheep IgG prepared against bovine brain calmodulin as shown by immunoblot. Rabbits and mice, challenged with the entire concretion organic fraction, produce an IgG fraction which always recognizes an epitope of the 17,000‐Da protein as the major antigenic determinant. This same protein is also a calcium binding protein as shown by 45Ca autoradiography even after heat and SDS/mercaptoethanol treatment. To determine that this protein was not bound to the concretions as an artifact of the isolation procedures, immunocytochemical procedures were used to localize the antigenicity to 17,000‐Da and vertebrate calmodulin in situ. Immunocytochemical localization indicates the protein is present on the concretions in situ but documents that the majority of this protein is located in concretion‐forming cells. Electron microscopic immunocytochemistry demonstrates that the protein is found largely in early concretion‐forming cells in protein granules formed before concretion architecture is observed. This may imply that the protein is likely important in the initiation of concretion formation, and is only a residual protein as found on the extracellular concretions.