The expression and distribution of the long form of Type XII collagen were investigated histochemically during chicken corneal development using a monoclonal antibody (P3D11) raised against the N-terminal domain of chicken Type XII collagen. Specificity of the antibody was confirmed by immunoprecipitation before and after bacterial collagenase digestion. Immunofluorescent microscopic studies showed that during chicken cornea formation, the long form of Type XII collagen is initially detected on Day 3 embryo (stage 19) in the sub-epithelial matrix of the corneal periphery and in the matrix around the optic cup. On Day 5 embryo (stage 27) the long form was expressed in the primary stroma. Thereafter, as the secondary stroma was formed, the long form localized in the sub-epithelial and sub-endothelial matrices and in the anterior region of the limbus (corneoscleral junction) before the formation of Descemet's and Bowman's membranes. After hatching, the immunoreactivity decreased predominantly in the sub-epithelial and sub-endothelial matrices but remained at the anterior region of the limbus. Immunoelectron microscopic examination demonstrated that the long form localizes in the Descemet's and Bowman's membranes and along the collagen fibrils in the stroma with a periodic repeat. Based on the distribution of the long form of Type XII collagen in the sub-epithelial and sub-endothelial matrices and limbus, it was suggested that the long form of Type XII collagen is involved in formation of the Descemet's and Bowman's membranes and in stabilization of the limbus.