The in vitro growth characteristics and morphology of human retinal pigment epithelium (RPE) cells from adult donor eyes (15 to 100 years of age) have been studied. Although RPE cells are viable in culture for several months, only a fraction of the cells actually divide. Rapid cell proliferation and confluency of a culture occur from 10 to 30 days after seeding. The time for confluency is both age- and media-dependent; the number of cells that are potential dividers and contribute to confluency decreases with increasing donor age. Since melanolipofuscin granules do not form in vitro and are diluted by cell division, stationary (nondividing) cells can be distinguished from the dividing cells by the presence of dense clusters of melanolipofuscin granules in the stationary cells. Confluent cultures contain a monolayer of relatively clear polygonal cells with densely pigmented stationary cells scattered throughout. Stationary cells can often represent as much as 95% of the original RPE cell population. Ultrastructurally all cells appear epithelioid, with apical-basal polarity, junctional complexes, and cytoplasmic organization characteristic of RPE cells in vivo. Stationary cells are extremely large, with melanolipofuscin granules clustered around the nucleus. Melanolipofuscin is a unique marker for nondividing cells and may facilitate studies of the age-dependent loss of replicability of human RPE.