AbstractTherapeutic drug monitoring allows to determine the best dosage regimen adapted to each patient optimizing the therapeutic benefits, while minimizing the risk for side effects. Here, the first methodological approach based on matrix‐assisted laser desorption/ionization source equipped with tandem time‐of‐flight (MALDI‐TOF/TOF) mass spectrometry for the determination of the antituberculosis (anti‐TB) drugs ethambutol, pyrazinamide, rifampicin, and streptomycin concentration in the plasma of tuberculosis‐infected patients is reported. The volume of the plasma sample was 200 μL. Plasma samples were cleaned‐up by protein precipitation and evaporated in a water bath under a nitrogen stream. The extracted samples were reconstituted with 200 μL of 50% methanol‐0.03% formic acid solution (v/v), spiked with known amounts of anti‐TB drugs, mixed (1:1) with a saturated matrix solution (4‐hydroxybenzoic acid in 50% acetonitrile‐0.1% trifluoracetic acid solution; v/v), and spotted onto the MALDI‐TOF/TOF sample target plate. The anti‐TB drug concentration was determined by standard additions analysis. Regression of standard additions was linear over the whole anti‐TB drug concentration range explored (the final anti‐TB drug concentration ranged from 0.20 to 200 pmol/μL). The absolute recovery of the anti‐TB drugs ranged between 87 and 110%. The minimal ethambutol, pyrazinamide, rifampicin, and streptomycin concentration detectable by MALDI‐TOF/TOF is 0.08, 0.20, 0.12, and 0.15 pmol/μL, respectively. © 2010 IUBMB IUBMB Life, 62(5): 387–393, 2010