The extrapallial fluid of Mytilus edulis is analyzed for inorganic and organic constituents. Disc gel electrophoresis reveals the presence of at least five protein components. The insoluble fraction is sulfated carbohydrate material while the ultrafiltrable portion contains free amino acids. The concentration of calclium is 9.8 ± 0.4 m m, 84.7% of which is complexed; 74.3% of the calcium is bound to small molecules, 9.2% to insoluble carbohydrate, 0.88% to soluble macromolecular components, and 15.3% is free. The data suggest that CaCO 3 deposition may be controlled by complexation of Ca 2+ by small chelates produced by the animal. EPR measurements indicate that the majority of manganese present in the fluid is also bound to small molecules. Titrations of the native fluid with Mn 2+ and H + establish that the chelating capacity of the fluid for Mn 2+ is nominally 10 −4 m. Mn 2+ forms 1:1 complexes in the fluid with ligands having an average p K a ′ value of 5.2