We systematically investigated how the therapeutic efficacy of a traceless, vascular targeting antibody–drug conjugate (ADC) is affected by the length of a spacer introduced between the antibody's globular fold and the site of drug attachment. Homogeneous ADCs were prepared from the direct conjugation of engineered C-terminal cysteines with a potent thiol containing drug which was separated from the antibody surface by unstructured spacers of increasing length. We found that a smaller spacer length is reflected in enhanced stability and therapeutic efficacy of the conjugates in a syngeneic model of murine cancer.