Abstract 1. 1. The proliferation of RIE-1 rat intestinal epithelial cells was potently but reversibly inhibited by transforming growth factor-β 1 (TGF-β 1). In early-passage cultures, complete growth arrest was observed when sparse cultures were treated with TGF-β 1 (1 ng/ml). 2. 2. However, increasing the initial cell culture density resulted in decreased TGF-β 1-mediated inhibition of cell proliferation. 3. 3. Independent of this population density effect, RIE-1 cells also exhibit a marked phenotypic transition around passage-8 to -10, such that later-passage cells were less responsive to growth inhibition by TGF-β 1].