A new oral cholecystopaque, iopronic acid, was compared to iopanoic acid. This was a double blind, randomized study of 260 subjects. A 4.5-G dose of iopronic acid produced a similar degree of gallbladder opacification as 3.0 g of iopanoic acid. Adverse reactions to the contrast media were generally mild, but were significantly less with iopronic acid. The present study is criticized because it was not designed to help the radiologist determine the better contrast medium under optimal clinical conditions.