An antibody-drug conjugate (ADC) consisting of the recombinant anti-epidermal growth factor receptor 2 (HER2) monoclonal antibody trastuzumab conjugated to the auristatin analog and potent microtubule inhibitor monomethyl auristatin F (MMAF), with potential antineoplastic activity. Upon administration of trastuzumab monomethyl auristatin F, the trastuzumab moiety binds to HER2 on tumor cell surfaces; upon internalization, the MMAF moiety binds to and inhibits tubulin polymerization, which results in G2/M phase arrest and tumor cell apoptosis. HER2, a tyrosine kinase receptor, is overexpressed by many cancer cell types.