The invention provides methods for labeling vitamin B12 with .sup.14C, .sup.13C, tritium, and deuterium. When radioisotopes are used, the invention provides for methods of labeling B12 with high specific activity. The invention also provides labeled vitamin B12 compositions made in accordance with the invention.
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
 This invention was made with Government support under Grant No. GM34804 awarded by the National Institutes of Health. The Government has certain rights in this invention. The United States Government also has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.