We have generated an antiserum to the variable domain of mouse collagen XXVII, a recently discovered novel member of the fibrillar collagen family. Collagen XXVII protein is first detectable in the mouse at embryonic day 12.5. By E14.5 days, the protein localises to cartilage, developing dermis, cornea, inner limiting membrane of the retina and major arteries of the heart. However, at E18.5 days, collagen XXVII protein is no longer apparent in most tissues and appears restricted mainly to cartilage where expression continues into adulthood. Type XXVII collagen immunolocalises to 10 nm thick non-striated fibrils that are distinct from fibrils formed by the classical fibrillar collagens. The transient nature of its expression and unusual fibrillar structure suggest that collagen XXVII plays a developmental role distinct from those of the classical fibrillar collagens.