An immunoconjugate consisting of an anti-CD19 monoclonal antibody conjugated to the maytansinoid DM4, a derivative of the cytotoxic agent maytansine (DM1), with potential antineoplastic activity. Anti-CD19-DM4 conjugate SAR3419 targets the cell surface antigen CD19, found on a number of B-cell-derived cancers. Upon antibody/antigen binding and internalization, the immunoconjugate releases DM4, which binds to tubulin and disrupts microtubule assembly/disassembly dynamics, resulting in inhibition of cell division and cell growth of CD19-expressing tumor cells.