Two papers in this issue of the Journal will surely give pause to physicians who by and large have come to expect that a routinely ordered "serum B12" will automatically tell them whether a patient is cobalamin deficient. Cooper and Whitehead report that serum levels of the vitamin are substantially higher when measured by the now widely available radioisotope dilution method than when determined by a more taxing microbiologic assay. More important, the microbiologic assay accurately identified 43 patients who, by other criteria, had well documented pernicious anemia, whereas "serum B12" levels measured by radioisotope dilution . . .