Prostate stem cell antigen (PSCA, named for its strong sequence homology to the thymocyte marker stem cell antigen 2) is a cell surface antigen expressed in normal prostate and associated with human and murine prostate cancer. To begin to investigate a possible link between PSCA expression in normal prostate and prostate carcinogenesis, we characterized the phenotype and proliferative behavior of normal PSCA-expressing prostate epithelial cells (PrEC) in tissue culture. PSCA was expressed in a subset of prostate epithelial cells that coexpress basal and secretory cytokeratins. PSCA-positive cells were the direct progeny of PSCA-negative cells and were characterized by a more differentiated morphology and a slower proliferative rate than PSCA-negative cells. Although PSCA-positive cells continued to express basal cell markers such as CD44, they lost expression of the basal cell marker p63. In contrast, expression of prostate specific antigen and androgen receptor transcripts was detectable in PSCA-positive PrEC. These findings suggest that PSCA is a unique marker of an intermediate subpopulation of PrEC in transition from a basal to a terminally differentiated secretory phenotype and may be a useful marker for the study of normal and malignant prostate development.