The effect of a purified fiber, psyllium and the short-chain organic acids derived from fiber breakdown, were assayed on isolated colonic epithelial cells primary-cultured from patients at high-risk of colon cancer, due to adenoma growth or family history of colon cancer. Psyllium fiber provided colonocytes some protection from deoxycholic acid-induced lysis. Propionic acid, a product of fiber breakdown, was a potent colonocyte mitogen, suggesting that fiber could indirectly protect the colon by providing colonocyte nutrients.