Alcoholic extracts of Trewia nudiflora seed have yielded several new maytansinoid tumor inhibitors that are exceptionally active in the PS, B1, and KB systems. These include trewiasine (1), dehydrotrewiasine (2), and demethyltrewiasine (3), which were characterized by NMR, mass spectral, and chemical correlation with other known maytansinoids. Two additional maytansinoids, trenudine (10) and treflorine (11), have been partially characterized. The Trewia ansa macrolides differ from others in this series in that they possess an additional methoxy group at C-15; and, with the exception of 1, they contain substituents at C-3 which differ from other known maytansinoids. Detailed 13C NMR assignments for maytansine and some related maytansinoids are presented.