The class III beta-tubulin isotype (beta III) is expressed exclusively by neurons within the normal human retina and is not present in normal retinal pigment epithelial (RPE) cells in situ or in 1 day-old primary cultures; however, beta III is present in RPE cells in 5-day primary cultures and in passaged RPE cells grown in monolayer cultures as determined by immunohistochemistry and Western blotting. beta III-positivity in cultured RPE cells is not affected by cell density or hydroxyurea- or retinoic-acid-mediated growth inhibition, but only a few cells weakly express beta III in cyclohexamide-treated cultures and RPE cells maintained in serum-free medium fail to produce beta III. When monolayer-cultured RPE cells grown in normal, serum-containing medium, are transferred to irradiated bovine vitreous, beta III is undetectable in most cells. Cultured RPE cells coexpress beta III with keratin and cellular retinaldehyde-binding protein (both RPE cell markers), but not with glial fibrillary acidic protein. Some cultured RPE cells also express neuron-specific (gamma) enolase, which is neuron-associated but not neuron-specific, and occasional cells in confluent or super-confluent cultures contain the 200-kDa neurofilament protein. Retinal glia, fibroblasts, endothelial cells, and smooth muscle cells do not express beta III under the same culture conditions. We have detected beta III in 45 of 56 epiretinal membranes, frequently in cells with a bipolar or dedifferentiated morphology, where its expression coincides with other RPE cell-associated antigens. Cells with morphological features resembling normal RPE cells in epiretinal membranes are usually negative for beta III, but RPE cells appearing to be in the early stages of dedifferentiation express the isotype weakly. Electron microscopic immunocytochemistry localizes beta III to microtubules, ribosomes and cytoplasm. beta III may be a useful marker for recognizing the fraction of RPE cells in epiretinal membranes that are no longer identifiable by morphological criteria or other RPE cell markers. These findings demonstrate that mature human RPE cells have the capacity to express a neuron-associated gene in response to conditions that promote dedifferentiation.