The metabolic pathways leading froml-[2-13 C]aspartic acid, [2-13 C]glycine andl-[methyl-13 C]methionine to vitamin B12were investigated, focusing on the biosynthetic pathways leading to the aminopropanol moiety of vita-min B12and on the role of the Shemin pathway leading tod-aminolevulinic acid (a biosynthetic intermediate of tetrapyrrole), by means of feeding experiments with Propionibacterium shermaniiin combination with 13 C-NMR spectroscopy.