Fatty acid synthesis from radiopropionate was evaluated in sural nerve biopsy slices from five normal controls and nine patients with pernicious anemia. The nerves were incubated in [(14)C]propionate, the lipids were extracted, and the fatty acid methyl esters were chromatographed by gas-liquid chromatography. In the normal nerves the radiolabel was found primarily in short chain (C12 and C14) fatty acids. The nerves from pernicious anemia patients showed two fatty acids peaks that were not discernible in the normal nerves, and these fatty acids had retention times intermediate to those of myristic (C14.0) and palmitic (C16.0) acids and palmitoleic (C16.1) and stearic (C18.0) acids, respectively. These two peaks (a C15 and C17 fatty acid) contained the bulk of the radioactivity recovered in the fatty acid fraction after incubation with [(14)C]propionate. Catalytic reduction and rechromatography failed to alter the retention time of these compounds suggesting that they are not unsaturated fatty acids. The nerves from the pernicious anemia patients had a decrease in the mean content of normal fatty acids when compared with the nerves from control patients as well as a decrease in the mean synthesis of normal fatty acids as estimated by isotope incorporation after incubation with [(14)C]propionate or (3)H(2)O. Analysis of myelin isolated from the nerves indicated that the changes at least in part were in that fraction.