For the study of the differentiation process of optic vesicle epithelium into neural retina, pigment epithelium and pars caeca retinae, vimentin intermediate filament protein in retinal epithelial cells was detected immunohistochemically in chick embryo at stages 11-21. In the late stage of optic vesicle development (stage 14), optic vesicle epithelium was classified into the following 3 different portions on the basis of vimentin staining intensity: latero-central epithelium under the lens placode, medio-central epithelium facing the latero-central epithelium, and peripheral epithelium connecting the latero-central and medio-central epithelia. Latero-central epithelium, the future neural retina, exhibited strongest staining of vimentin of the 3 portions. In contrast, medio-central epithelium, the future pigment epithelium, showed weakest staining. Moderate staining was observed in peripheral epithelium, the future pars caeca retinae. These differences in levels of vimentin expression were observed during optic cup formation. The present results clearly demonstrate that differentiation of retinal epithelium into neural retina, pigment epithelium and pars caeca retinae occurs in the late stage of the optic vesicle, and that retinal differentiation is reflected by the amount of vimentin in epithelial cells.