Three preparations, two based on magnesium citrate and one an optimised oral mannitol regime, have been compared for their effectiveness in clearing the large bowel prior to double-contrast barium enema and for effects on barium mucosal coating. If excellent clearance alone is taken as the criterion of acceptability, the preparations based on magnesium citrate are shown to be equal to each other, but better than mannitol. No significant difference in coating performance is seen between the three preparations. Magnesium citrate regimes cause significantly less nausea and vomiting than mannitol. No differences are noted in the frequency of abdominal or rectal pain between the preparations. It is concluded that mannitol should be abandoned as a barium enema preparation.