Undissociated tissue explants of the retina and retinal pigment epithelium (RPE) of 3,5-, 4-, 5- and 8-day-old chick embryos were cultured in vitro. After 7 days in culture, lentoids were observed in explants of either retina or RPE from 3,5-, 4- and 5-day-old embryos. As demonstrated by immunohistochemistry, these lentoids contained specific chick lens proteins (alpha-, beta- and delta-crystallins). No crystallin-containing cells were found in eye tissue explants from 8-day-old embryos. However, when 5-bromo-deoxyuridine (25 microM) was introduced into the medium at the beginning of culturing (for 12 h), large eosinophilic cells containing alpha-, beta- and delta-crystallins were detected in retinal explants of the 8-day old embryos. Thus, retina and RPE of 3,5-5-day-old chick embryos are capable of lens differentiation after explantation in vitro without dissociation into individual cells. This capacity is lost during development.