The antitumor agent taxol has been examined by electron ionization, chemical ionization, and fast atom bombardment mass spectrometry. Three ion series are observed: (1) the M-series, characteristic of the intact molecule; (2) the T-series, with fragments derived from the taxane ring; and (3) the S-series representing the C-13 side chain. Neutral losses dominate each series of ions and serve to verify the presence and number of functionalities in each portion of the molecule. Fragmentation pathways and mechanisms of ion formation are proposed on the basis of product ion analysis and accurate mass measurements.