Four patients had serum cobalamin levels that were initially thought to be falsely low. None of the patients had the typical hematologic and clinical features of cobalamin deficiency and the three so tested had normal cobalamin absorption and/or gastric function. However, the deoxyuridine suppression test result was abnormal in all four cases. The abnormality was improved by adding cobalamin in vitro but, in three of the four cases, was not corrected and, indeed, was made worse by the addition of methyl tetrahydrofolate. These results established metabolically the presence of cobalamin deficiency. The recently improved cobalamin radioassays may be unmasking some atypical and, heretofore, unappreciated cobalamin-deficiency states (only one of our four patients would have come to attention with the older radioassays). These can be identified with the deoxyuridine suppression test, particularly when specific in vitro additives are included.