Accelerator mass spectrometry (AMS) is an ultra-sensitive analytical method suitable for the detection of sub-nM concentrations of labeled biological substances such as pharmaceutical drugs in body fluids. A limiting factor in extending the concentration measurements to the sub-pM range is the natural 14C content in living tissues. This was circumvented by separating the labeled drug from the tissue matrix, using standard high-performance liquid chromatography (HPLC) procedures. As the separated total drug amount is in the few fg range, it is not possible to use a standard AMS sample preparation method, where mg sizes are required. We have utilized a sensitive carbon carrier method where a 14C-deficient compound is added to the HPLC fractions and the composite sample is prepared and analyzed by AMS. Using 50 µL human blood plasma aliquots, we have demonstrated concentration measurements below 20 fM, containing sub-amol amounts of the labeled drug. The method has the immediate potential of operating in the sub-fM region. Copyright © 2009 John Wiley & Sons, Ltd.