Conjugation of cytotoxic compounds to antibodies that bind to cancer-specific antigens makes these drugs selective in killing cancer cells. However, many of the compounds used in such antibody-drug conjugates (ADC) are substrates for the multidrug transporter MDR1. To evade the MDR1-mediated resistance, we conjugated the highly cytotoxic maytansinoid DM1 to antibodies via the maleimidyl-based hydrophilic linker PEG(4)Mal. Following uptake into target cells, conjugates made with the PEG(4)Mal linker were processed to a cytotoxic metabolite that was retained by MDR1-expressing cells better than a metabolite of similar conjugates prepared with the nonpolar linker N-succinimidyl-4-(maleimidomethyl)cyclohexane-1-carboxylate (SMCC). In accord, PEG(4)Mal-linked conjugates were more potent in killing MDR1-expressing cells in culture. In addition, PEG(4)Mal-linked conjugates were markedly more effective in eradicating MDR1-expressing human xenograft tumors than SMCC-linked conjugates while being tolerated similarly, thus showing an improved therapeutic index. This study points the way to the development of ADCs that bypass multidrug resistance.