Methylmalonic acid concentrations are increased in serum in vitamin B12 (cobalamin) deficiency. Here I demonstrate the successful use of anion-exchange extraction for improving newly developed analytical procedures and describe well-documented, reliable performance of this method for rapid determination of methylmalonic acid. The sorbent counter ion is formate, and the elution solvent is formic acid. The dicyclohexyl derivative is measured by selected ion monitoring. For serum, the assay curve is linear from 0.026 to 200 mumol/L. The normal reference interval is 0.08 to 0.56 mumol/L. Added methylmalonic acid is accurately quantified. The sensitivity and the precision exceed those of the current method by three orders of magnitude. The total and within-day CVs are 4.6% to 7.9% and 2.6% to 4.7%, respectively. Similar figures were obtained for urine. This convenient method is useful for evaluation of cobalamin deficiency, especially in patients with normal or moderately depressed cobalamin concentrations in serum.