A new biosynthetic pathway, which can produce both vitamin B-12 and large amounts of porphyrins from isopropanol, was identified in Arthrobacter hyalinus using carbon-13 stable isotope tracer techniques and carbon-13 nuclear magnetic resonance (C-13-NMR) spectroscopy. Studies on the incorporation of [2-C-13]isopropanol, [1- or 2-C-13]sodium acetate, L-[1-C-13]glutamate, and [1-, 2-, 3-, 4-, 5-C-13]S-aminolevulinic acid into uroporphyrinogen III showed that isopropanol was metabolized into uroporphyrinogen III through acetyl CoA and that 5-aminolevulinic acid was produced from L-glutamic acid and not via Shemin’s pathway.