Extracts of Treuia nud&ra (Euphorbiaceae) seed have previously yielded six new maytansinoids. These compounds, the most abundant of which is trewiasine (l), have potent tumor-inhibiting and insect-control properries in experimental systems. Further work has revealed two additional members of this series; 10-epitrewiasine (7) and nortrewiasine (8). Colubrinol(9), previously known only as a constituent of Colubrina texensis (Rhamnaceae), was also present. Isolation of the new maytansinoids was achieved by hplc, and structures were elucidated by 'H-nrnr, 13C-nmr, and mass spectroscopy. In contrast to results with electron impact mass spectrometry, observation of molecular ions was achieved using negative ion chemical ionization mass spectrometry. Application of this technique to a series of previously characterized maytansinoids revealed that all give significant molecular anions.