Lysosomal cysteine proteases possess the potential to activate certain pro-drugs, once these have entered the lysosomal compartment. An important example of this concept is the use of antibody-drug conjugates (ADCs) designed to include a cathepsin-cleavable linker. To achieve targeted drug delivery with this strategy, we have designed an ADC based on an antibody against a recycling, endocytic receptor on sarcoma cells with limited expression in the normal adult organism. Using fluorescence-labeled reagents, we could show that this antibody is routed to the lysosomes of receptor-positive cells, following clathrin-dependent endocytosis. The current ADC includes the potent toxin, monomethyl auristatin E (MMAE) which is coupled to the antibody via a cathepsin-labile (valine-citrulline) linker, using a well-established format also employed for other ADCs. In vitro, this ADC kills receptor-positive cells in the sub-nanomolar range with very low toxicity against receptor-negative cells. Lysosomal drug release was crucially important for the toxic effect since inhibition of lysosomal cysteine-type cathepsins led to protection against toxicity in receptor-positive cells. When used against a transplanted mouse tumor model with receptor-positive tumor cells, local administration of this ADC led to apparent eradication of palpable tumors in 9 out of 10 mice with no evident systemic toxicity. 5 mice remained tumor negative after termination of treatment, whereas tumor regrowth was observed in 4 mice. No effect was obtained with an ADC designed in the same manner but including an irrelevant antibody. Although the importance of protease-sensitive linkers has been questioned in connection with other ADCs, our findings demonstrate the strength of targeting recycling endocytic receptors using prodrugs dependent on lysosomal release. These findings are important for the further development of a generalized strategy for targeted drug delivery.