New thiol and disulfide-containing maytansinoids bearing a mono or di-alkylsubstitution on the .alpha.-carbon atom bearing the sulfur atom are disclosed.Also disclosed aremethods for the synthesis of these new maytansinoids and methods for thelinkage of thesenew maytansinoids to cell-binding agents. The maytansinoid-cell-binding agentconjugatesare useful as therapeutic agents, which are delivered specifically to targetcells and arecytotoxic. These conjugates display vastly improved therapeutic efficacy inanimal tumormodels compared to the previously described agents.
