New thiol and disulfide-containing maytansinoids bearing a mono or
di-alkyl substitution on the .alpha.-carbon atom bearing the sulfur atom
are disclosed. Also disclosed are methods for the synthesis of these new
maytansinoids and methods for the linkage of these new maytansinoids to
cell-binding agents. The maytansinoid-cell-binding agent conjugates are
useful as therapeutic agents, which are delivered specifically to target
cells and are cytotoxic. These conjugates display vastly improved
therapeutic efficacy in animal tumor models compared to the previously
described agents.