An immunoconjugate consisting of a monoclonal antibody directed against a highly-expressed myleoma cell surface antigen covalently attached to the maytansinoid DM4, a derivative of the cytotoxic agent maytansine (DM1), with potential antineoplastic activity. Anti-myeloma monoclonal antibody-DM4 immunoconjugate BT-062 binds to an unspecified cell surface antigen highly expressed on myeloma cells; upon internaliization the DM4 moiety is released, binding to tubulin and disrupting microtubule assembly/disassembly dynamics, which may result in the inhibition of cell division and cell growth of myeloma tumor cells.