The mucous layer located on the cornea epithelial surface is mainly composed of mucin.It is unique in being composed of a central protein back bone surrounded by an outermost dense oligosaccharide matrix, yet its pre-cise composition is still unknown. Conjunctival goblet cells have been known to express the mucin MUC5AC. We attempted to reveal the presence of the secretory mucin MUC5AC as a component of the mucous layer in the rabbit corneal tear film. An antibody to a MUC5AC peptide was used to detect the MUC5AC mucin present in the tear film by immunohistochemistry. Human conjuncti-val impression cytology and immunohistochemistry were done with the same antibody. The MUC5AC mucin was observed in the precorneal tear film and con-junctival goblet cells by immunohistochemistry.The MUC5AC mucin is a component of the mucous layer and is produced by the goblet cells of con-junctiva.