The ciliary epithelium is a bilayer of epithelial cells responsible for the formation and secretion of aqueous humor in the mammalian eye. We have isolated a cDNA clone from a lambda gt11 cDNA library of bovine ocular ciliary epithelial cells encoding the CD9 antigen, a member of a new family of transmembrane proteins. The bovine CD9 clone contains an open reading frame of 226 amino acids (M(r) 24,860). The deduced amino acid sequence from the bovine CD9 cDNA clone shows 83.5% identity with the human counterpart isolated from megakaryocytes, and a lower degree of identity with a group of related antigens (TAPA-1, C0-029, CD53, MRC OX-44, ME491, CD63, CD37, and Sm23) involved in growth regulation. Analysis of bovine ocular tissues reveals that the CD9 gene encodes a 1.4 kb mRNA which is detectable predominantly in cornea and at low levels in ciliary epithelium, retina, iris, and lens. Normal and transformed cell lines established from the ocular ciliary epithelium exhibited significant levels of CD9 transcripts. These results raise questions regarding possible roles of CD9 in the anterior segment of the eye.