Intratumoral MMAE concentrations consistently correlates with the extent of tumor growth inhibition in tumor xenograft models. IHC analysis reveals that nonbinding control-treated tumors consist of both CD30 ⁺ and CD30 ^- cells, presumably because they do not kill either CD30 ⁺ or CD30 ^- Karpas 299 cells. Only CD30 ^- cells are found in cAC10-vcMMAF-treated tumors, illustrating that cAC10-vcMMAF eliminates most CD30 ⁺ cells. Interestingly, the two tumors that relapses from cAC10-vcMMAE treatment are also found to be CD30 ^- by the end of study, indicating a small fraction of CD30 ^- cells might have escaped from bystander killing in these two remaining tumors.