A series of new taxoids derived from 14 beta-hydroxy-10-deacetylbaccatin III was synthesized by means of the beta-lactam synthon method. Most of the new taxoids thus synthesized possess excellent cytotoxicity against human ovarian (A121), non-small-cell lung (A549), colon (HT-29), and breast (MCF-7) cancer cell lines, and several of these taxoids show subnanomolar IC50 values which are severalfold to 1 order of magnitude better than those of paclitaxel and docetaxel. Modifications at the 3'- and 3'-N-positions exert marked effects on the activity. For the substituents at C-3', the cytotoxicity decreases in the order 2-furyl approximately 2-methyl-1-propenyl > or = 2-methylpropyl > (E)-1-propenyl > or = n-propyl > phenyl > > 2,2-dimethylpropyl. For the 3'-N substituents, the activity decreases in the order t-BuOCO > Ph > n-hexanoyl. A significant increase in the cytotoxicity against the doxorubicin-resistant human breast cancer cell line MCF7-R that expresses the multidrug resistance (MDR) phenotype is observed by the proper modification of the substituent at C-10. The observed remarkable effects of the substituents at C-10 on the activity against MCF7-R can be ascribed to the effective inhibition of the binding of these new taxoids to P-glycoprotein that is responsible for MDR.