Oral doses of either iopronic acid (4.5 g Oravue, Squibb) or iopanoic acid (3 g Telepaque, Winthrop) were given to 98 patients requiring cholecystography. Radiographs were taken 13 to 16 hours after treatment showed good to excellent gallbladder opacification in 44 percent of patients after the first dose of iopronic acid and in an additional 29 percent after a second dose. Similar opacification occurred in 42 percent of patients after the first dose of iopanoic acid and in 34 percent after a second dose. Drug-related abnormalities in blood and urine tests occurred about equally in both groups and one patient in each group exhibited a clinically adverse reaction (diarrhea). Thus, the performance (radiographic efficacy and drug safety) of the new contrast agent, iopronic acid, was similar to a widely used drug, iopanoic acid.