The role of avian eggshell matrix proteins in shell formation is poorly understood. This calcitic biomaterial forms in a uterine fluid where the protein composition varies during the initial, calcification, and terminal phases of eggshell deposition. A specific antibody was raised to a 116-kDa protein, which is most abundant in uterine fluid during active eggshell calcification. This antiserum was used to expression screen a bacteriophage cDNA library prepared using mRNA extracted from pooled uterine tissue harvested at the midpoint of eggshell calcification. Plasmids containing inserts of differing 5′-lengths were isolated with a maximum cDNA sequence of 2.4 kilobases. Northern blotting and reverse transcriptase-polymerase chain reaction demonstrated that the 2.35-kilobase message was expressed in a uterine-specific manner. The hypothetical translational product from the open reading frame corresponded to a novel 80-kDa protein, which we have named ovocleidin-116. After removal of the predicted signal peptide, its N-terminal sequence corresponded almost exactly with that determined from direct microsequencing of the 116-kDa uterine protein (this work) and with that previously determined for the core protein of a 120-kDa eggshell dermatan sulfate proteoglycan (Corrino, D. A., Rodriguez, J. P., and Caplan, A. I. (1997) Connect. Tissue Res. 36, 175–193). Ultrastructural colloidal gold immunocytochemistry of ovocleidin-116 demonstrated its presence in the organic matrix, in small vesicles found throughout the mineralized palisade layer, and the calcium reserve assembly of the mammillary layer. Ovocleidin-116 thus is a candidate molecule for the regulation of calcite growth during eggshell calcification.