Because virtually all cases of vitamin B12 deficiency seen in this country are due to malabsorption, the availability of radioactive vitamin B12 for direct measurement of absorption of this essential nutrient has proved to be of great clinical value. These tests are useful not only in demonstrating vitamin B12 malabsorption but also often in defining the pathophysiological mechanism responsible for this abnormality. The urinary excretion test of Schilling remains the most useful test for vitamin B12 absorption. Minor precautions and modifications in technique make the test results more reliable and easier to interpret. The 8-hr plasma test for vitamin B12 absorption can no longer be considered acceptable. Some patients with vitamin B12 malabsorption have results in the normal range when studied by this method. Serum vitamin B12 assays utilizing radioactive vitamin B12 and the isotope dilution principle are not widely used and are useful screening tests. Low normal or borderline results observed in patients with clinical evidence suggestive of vitamin B12 deficiency should be interpreted with caution or confirmed by radioactive vitamin B12 absorption studies. Radioactive vitamin B12 can also be used for rapid, reliable assay of gastric intrinsic factor, antibody to intrinsic factor and unsaturated vitamin B12 serum. Methods using radioactive folate compounds for similar in vivo and in vitro studies are not yet applicable for routine use in nuclear medicine laboratories.