Despite the widespread use of 14C-labeled compounds in clinical research, the Atomic Energy Commission's current list of "recognized medical uses of radioisotopes" does not include a single test using 14C. But the breath test for measuring bacterial deconjugation of bile acids may soon qualify as the first routine clinical diagnostic procedure using 14C. Since the test was reported 2 years ago (1, 2), three other laboratories (3, 4)1 have agreed that the test can give useful diagnostic results in enterohepatic disease. The test is appealing because it is simple to do and to interpret. The procedure is noninvasive, has no