A simple, rapid liquid chromatography/tandem mass spectrometric (LC–MS/MS) assay was developed and validated for the quantification of both unbound and total paclitaxel in plasma following treatment with Abraxane (ABI-007) or Taxol. Accurate and reproducible analysis of ABI-007, an albumin nanoparticle formulation of paclitaxel could not be achieved using previously published methodology designed for Taxol. The final validated method involved protein precipitation followed by vacuum filtration, in a 96-well format for rapid processing. The 4 min run employed gradient elution on a Waters SymmetryShield C8 (2.1 mm × 50 mm, 3.5 μm) column, followed by tandem mass spectrometric detection, in electrospray positive mode. Calibrator samples were prepared daily with paclitaxel and analyzed with both ABI-007 and paclitaxel quality control samples. To measure unbound drug, sample preparation was preceded by ultrafiltration. The assay was linear over the range of 10–2500 ng/mL, with dilution providing measurement up to 50,000 ng/mL. Within-run and between-run precision for all QC samples was less than 5.0% and 10.4%, respectively. Accuracy was high, with deviation of less than 6.1% for all QCs. Measurement of unbound paclitaxel was precise (BRP and WRP <10%).