Radioisotopic contamination by recent radioisotope administration (either 99mtechnetium or 67gallium) to the patient for scanning purposes was found in 28 sera submitted for vitamin B12 or folic acid assay, or both, during the year prior to this study. In half the cases serum radioactivity was so great as to be readily identifiable during the assay. In the rest, however, the radioactivity would not have been detected had serum supernatant blanks not been routinely incorporated in the assay procedure. Many radioassay kits make no provision for such supernatant control blanks. Were it not for these blanks, five patients would have been falsely identified as vitamin B12-deficient and three others as possible deficient in the present series. Two of the five radioactive sera assayed for folate would also have given misleading results in the borderline-low range. The clinician should avoid ordering tests requiring radioassay when the patient has been given radioisotopes, and those performing radioassays must institute appropriate maneuvers to detect any such contamination.