Abstract: The present invention relates to the use of a Shiga toxin B-subunit moiety as carrier for therapeutic agents, for example, anti-cancer agents such as anti-cancer agents that require intracellular uptake to exert their anti-cancer effects. In particular, the present invention provides conjugates comprising a Shiga toxin moiety covalently linked to an anti-cancer agent through a self-immolative spacer, and methods of using such conjugates to increase cellular uptake and/or specificity for cancer cells of the anti-cancer drug. Also provided are methods of treatment involving administration of such conjugates, and pharmaceutical compositions and kits useful for carrying out such methods of treatment.