Abstract
Sera were absorbed with polyacrylamide beads to which purified human intrinsic factor was attached. This procedure removed the vitamin B12 analogs which were measured by nicrobiological assay with Lactobacillus leichmannii and Euglena gracilis and in an isotope dilution method using intrinsic factor. Such sera still contained B12 analogs that were assayed in an isotope dilution method using a nonintrinsic factor vitamin B12 binder. Such vitamin B12 analogs make up .apprx. 1/2 of the total vitamin B12 analogs in human serum.