The absorption of 10 mug doses of vitamin D(3)-(3)H in rats was found to be decreased by the addition of cholestyramine to the diet in an amount sufficient to cause steatorrhoea. Studies in vitro suggested that the effect of cholestyramine was due to its known ability to bind bile salts and thus disrupt micelles containing vitamin D. The absorption of 1 mg (47)calcium chloride was similar in both control and cholestyramine-fed rats, whether estimated by the whole body counting or the faecal excretion technique. These findings show that the administration of cholestyramine causes a reduction in the absorption of vitamin D(3)-(3)H but not of radiocalcium.