The present invention provides novel N-substituted hydrazine bifunctional compounds, novel N-substituted hydrazone derivatives of a cytotoxic reagent incorporating the bifunctional compounds, novel conjugates containing at least one cytotoxic reagent molecule reacted with the bifunctional compound and bound to a molecule reactive with a target cell population, methods for their production, and pharmaceutical compositions and methods for delivering cytotoxic reagents to a target population of cells. The hydrazone bonds of the conjugates of the invention permit the release of free cytotoxic reagent from the conjugates in the acidic external or internal environment of the target cells. The bifunctional compounds, derivatives, conjugates and methods of the invention are useful in antibody-or ligand-mediated drug delivery systems for the preferential killing of a target cell population to treat diseases such as cancers, infections and autoimmune disorders.