Cytotoxic T-lymphocyte antigen-2 alpha (CTLA-2α) has been discovered and expressed in mouse activated T-cells and mast cells. Structurally, it is homologous to the proregion of mouse cathepsin L, a lysosomal cystein proteinase. Expressed recombinant CTLA-2α is shown to exhibit selective inhibition to cathepsin L and is localized in the uterus during pregnancy where it is implicated in embryo plantation and development. CTLA-2α has also been demonstrated in the maternal side of the placenta in the deciduas basalis, metrial gland and myometrium layers in mouse but its vivo targets in the embryo are yet to be identified. We carried out studies to investigate the specific cell types synthesizing CTLA-2α protein in mouse embryo and examine its cellular localization. Immunofluorescence labeling showed intense localization of CTLA-2α in the cranium, vertebrae of cervical and thoracic region and the sternabrae. In the visceral organs, staining level was strong in the pancreas. Moderate staining was visible within the brain and remnants of the notochord. The rest of the organs including the spleen, small intestine and lungs were delineated by CTLA-2α. These findings suggest that CTLA-2α participates in an important role from the potential commitment of mesenchymal cells lineages to the ossification of axial skeleton early in embryogenesis.