Methylmalonic acid (MMA), a biochemical marker for vitamin B12 deficiency, is commonly measured in serum and urine using liquid chromatography-tandem mass spectrometry (LC-MS-MS) with electrospray ionization. Improvements in sample preparation and the ability to analyze MMA without derivatization have further simplified the analysis. In the method we describe, 200 microl of plasma spiked with deuterated d3-MMA internal standard is deproteinized using ultrafiltration. After acidification, the ultrafiltrate is injected into the LC-MS-MS system and MMA eluted under isocratic conditions. MMA and d3-MMA are monitored in multiple reaction monitoring (MRM) in the negative ion mode.