Clusterin is a widely expressed secretory glycoprotein which is found in mammals as a disulfide-bonded alpha/beta heterodimer generated by cleavage of the single-chain precursor. In contrast, clusterin occurs in the chicken mainly as an intracellular single-chain form and is not observed in serum. The present report identifies chicken clusterin as a component of the eggshell. This extracellular clusterin originates in the uterine fluid, where it is a disulfide-bonded heterodimer derived from the precursor polypeptide by proteolytic cleavage at the same site as in mammals. Clusterin message expression in the oviduct was measured by real time RT-PCR, and levels were found to be highest in magnum and uterus. Western blotting using protein extracts of oviduct tissues indicated major clusterin production in the magnum, while immunostaining of the oviduct identified clusterin in the tubular glands of the uterus and the magnum. In addition, clusterin was detected in egg white by Western blotting. In the decalcified eggshell, immunofluorescence and colloidal-gold immunocytochemistry revealed that clusterin was predominantly localized in the palisade and mammillary layers, but also in the mantle and core of the inner and outer shell membranes. It has been suggested recently that clusterin acts as an extracellular chaperone. Thus clusterin could function in the uterine fluid to prevent the premature aggregation and precipitation of eggshell matrix components before and during their assembly into the rigid protein scaffold necessary for ordered mineralization.