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Bile of the rat and of other species stimulates the uptake of β-carotene from a micellar solution of Tween 40 (polyoxyethylene anhydrosorbitol monopalmitate) and its cleavage into retinol ester by intestinal sections of the rat incubated in vitro. Sodium glycocholate and several other conjugated bile acids substitute completely for bile. Glycocholate also stimulates β-carotene cleavage by intestinal sections of the chicken, hamster, and lamb, but not of the turtle or opossum.The stimulatory effect of several bile acids tested in a medium containing Tween 40 is roughly proportional to the number of hydroxyl groups present. With the exception of cholic acid, only conjugated bile acids have this activity. The binding of β-carotene to the intestine occurred rapidly, and could not be readily correlated with the formation of retinol ester.Although the physicochemical state of carotene in solution is undoubtedly important, the conjugated bile acids probably stimulate the formation of retinol ester by enhancing the absorption of β-carotene by means of an interaction with the membrane of intestinal mucosal cells.