The expression of the β_1C integrin, an alternatively spliced variant of the β₁ subunit, was investigated in human adult and fetal tissues. In the adult, β_1C immunoreactivity was found in nonproliferative, differentiated simple, and/or pseudostratified epithelia in prostate glands and liver bile ducts. In contrast, β_1C was undetectable in stratified squamous epithelium of the epidermis and/or in hepatocytes. Luminal prostate epithelial cells expressed β_1Cin vivo and in vitro, but no β_1Cwas seen in basal cells, which are proliferating cells. Fetal prostate expressed β_1C in differentiated glands that had a defined lumen, but not in budding glands, indicating that β_1C is a marker of prostate epithelium differentiation. The β_1C and the common β_1Avariants are differentially distributed: β_1A was found in luminal and basal epithelial as well as in stromal cells in the prostate. In the liver, β_1C and β_1Awere coexpressed in biliary epithelium, whereas vascular cells expressed only β_1A. Because we found β_1C in nonproliferative and differentiated epithelium, we investigated whether β_1C could have a causal role in inhibiting epithelial cell proliferation. The results showed that exogenous expression of a β_1C, but not of a β_1A, cytoplasmic domain chimeric construct, completely inhibited thymidine incorporation in response to serum by prostate cancer epithelial cells. Consistent with these in vitro results, β_1C appeared to be downregulated in prostate glands that exhibit regenerative features in benign hyperplastic epithelium. These data show that the presence of β_1C integrins in epithelial cells correlates with a nonproliferative, differentiated phenotype and is growth inhibitory to prostate epithelial cells in vitro. These findings indicate a novel pathophysiological role for this integrin variant in epithelial cell proliferation.