An immunoconjugate consisting of an anti-integrin monoclonal antibody covalently attached to the maytansinoid DM4, a derivative of the cytotoxic agent maytansine (DM1), with potential antineoplastic activity. Anti-integrin monoclonal antibody-DM4 immunoconjugate IMGN388 binds to tumor cell surface integrins; upon internalization, the DM4 moiety is released from the immunoconjugate, binding to tubulin and disrupting microtubule assembly/disassembly dynamics, which may result in inhibition of cell division and cell growth of integrin-expressing tumor cells. Integrins, a class of transmembrane cell surface receptors, link the extracellular matrix (ECM) to intracellular signaling pathways that control cell proliferation and differentiation.