3,7,8-15 N 3 -N 1 -(b-D-erythro-pentofuranosyl)-5-guanidinohydantoin was synthesized from the oxidation of 1,7,NH 2 -15 N 3 -8-oxo-7,8-dihydro-2 0 -deoxyguanosine with 2 equivalents of Ir(IV) in pH 4.5 potassium phosphate buffer. The synthesis of 1,7,NH 2 -15 N 3 -8-oxo-7,8-dihydro-2 0 -deoxyguanosine started with bromination of 1,7,NH 2 -15 N 3 -2 0 -deoxyguanosine. The resulting 1,7,NH 2 -15 N 3 -8-bromo-7,8-dihydro-2 0 -deoxyguanosine reacted with sodium benzyloxide to afford 1,7,NH 2 -15 N 3 -8-benzyloxy-7,8-dihydro-2 0 -deoxyguanosine. Subsequent catalytic transfer hydrogenation of 1,7,NH 2 -15 N 3 -8-benzyloxy-7,8-dihydro-2 0deoxyguanosine with cyclohexene and 10% Pd/C yielded 1,7,NH 2 -15 N 3 -8-oxo-7,8-dihydro-2 0 -deoxyguanosine. Purification of 3,7,8-15 N 3 -N 1 -(b-D-erythropentofuranosyl)-5-guanidinohydantoin was first carried out on a C18 column and the product was further purified on a graphite column. ESI-MS was used to confirm the identity and to determine the isotopic purity of all the labeled compounds. The isotopic purity of 3,7,8-15 N 3 -N 1 -(b-D-erythropentofuranosyl)-5-guanidinohydantoin was 99.4 atom% based on LC-MS measurements.