In 36 patients with Addisonian pernicious anemia and in 24 control patients, measurements were made of the percentage of an orally administered dose of radioactive vitamin B12 Co60 that appeared in the urine. In the performance of the test in addition to the radioactive vitamin B12, 1000 [mu]g of non-radioactive vitamin B12 was given orally. Patients with pernicious anemia excreted less than 3.0% range (0.05-3.0%)-mean 0.85%, and the controls excreted more than 5% (range 5.0 to 34.2%), mean 13.3%. Significant excretion was found in the second 24 hour collection period in the control group.