Bivalent doxorubicin (DOX)-dipeptides ( 16a – c ) were prepared and conjugated to the monoclonal antibody BR96. The dipeptides are cleaved by lysosomal proteases following internalization of the resulting immunoconjugates. Conjugate 18b demonstrated antigen-specific in vitro tumor cell killing activity (IC 50 =0.2 μM) that was equipotent to DOX with a near doubling of drug molecules/MAb. Size exclusion chromatography showed 18b to be a noncovalent dimer that was formed immediately upon conjugation.