It is not known whether the differentiated fate of retinal precursor cells is determined before, during, or after terminal mitosis. Previous studies from this laboratory led to the hypothesis that retinal precursor cells remain plastic after final mitosis and will follow a photoreceptor "default pathway" unless induced to develop as neurons by intraretinal factors. This hypothesis predicts that isolated precursors undergoing terminal mitosis and differentiation in cell culture, in the absence of the retinal microenvironment, should become photoreceptors, regardless of embryonic age. To test this prediction precursor cells were dissociated from 5- to 8-day chick embryo retinas and grown as single cells in vitro. Bromodeoxyuridine (BRDU)- and [3H]thymidine-labeling techniques, coupled with serial photography of precursor development in culture, showed that at all donor ages some of the isolated cells divided one or more times and became postmitotic in vitro. Analysis of cell phenotype by phase-contrast microscopy, sequential photography, autoradiography, and immunocytochemistry showed that the majority of precursors from all donor ages differentiated as photoreceptors. These observations support a prediction derived from the "photoreceptor default" hypothesis.