The 55 kDa keratin K12 and the 59 kDa keratin K4 were used as biochemical markers of differentiated corneal and conjunctival epithelium, respectively, to follow the temporal and spatial appearance of these cell types during embryonic development of the mouse eye. K12 was first detected in corneal epithelial cells of day 15 mouse embryos in a small subpopulation of superficial cells. At later developmental stages only suprabasal corneal epithelium expressed K12, however, in post-natal and adult cornea all cell layers were K12-positive. K4 was first observed, in 14 and 15 day embryos, in a subpopulation of epithelial cells which had invaginated from surface ectoderm to form the lid buds. From embryonic day 16 on K4 was detected in all areas of developing conjunctival epithelium. Some superficial corneal epithelial cells also expressed K4 during embryonic development, but by immunofluorescence microscopic criteria, this keratin was localized exclusively to the conjunctiva in post-natal and adult eye. Expression of the 50 kDa 'basal-type' keratin K14 was also examined in this study. Similarly to K4, K14 was first noted in epithelium comprising the lid bud at embryonic day 14. Between 14 and 17 days of development some epithelial cells in the putative fornix of the conjunctiva did not express K14. Although corneal epithelial cells expressed K14 during development, in adult cornea only certain basal cells did so. These results suggest that the invagination of surface ectoderm to form the presumptive eyelid may be coupled to the initiation of differentiation of ocular surface epithelium.