Theϵ-crystallin polypeptide is a recently described protein in the eye lens of the peking duck,Anas platyrhynchos(1). It does not cross react withα-,β-,δ-, andγ-crystalli.ns.It has a molecular weight of 120K and consists of 3 identical 38K polypeptides. It is found in some reptiles and birds.The ontogeny of this polypeptide has been investigated in the developingA.platyrhynchoslens via the indirect immunofluorescence staining using a homologous antibody.The 38K. polypeptide was extracted from 13% Tris-SDS acrylamide gels (3), lyophilized and injected into a young rabbit to raise an antibody.The purity of the isolated 38K polypeptide and and the specificity of the antibody were checked by Tris-SDS gel electrophoresis and immunoblotting, respectively.The first positive reaction is detected at 80 h (stage 18) incubated lens. It is confined to a few elongating early primary fibres.Until the 9th day of development the reaction is confined to the primary and secondary lens fibres, The first positive reaction in the annular pad area is observed in the“day 10”lens.In the anterior epithelium the first positive reaction is detectable in the“day 12”lens. At the beginning it is confined to a few cells in the center of the epithelium and gradually the reaction spreads to other cells. A strong and uniform reaction in the entire epithelium is noted for the first time in the lens of a just-hatched duckling.The 38Kϵ-polypeptide is detectable after theα-,β-, andδ-crystallins, which, in the duck, appear simultaneously from 66 h (stage 15/16) (6).