Abstract: The invention relates to anthracycline conjugates comprising at least one anthracycline molecule linked to a molecule that is reactive with a cell population to be eliminated such as antibody, bombesin, EGF and transferrin. Each anthracycline molecule, having a keto group at the C-13 position, is conjugated to the antibody via a linker arm and is bound to that linker arm via an acid-sensitive acylhydrazone bond at the 13-keto position of the anthracycline. The linker additionally contains a disulfide or thioether linkage as part of the antibody or ligand attachment to the immunoconjugate. The novel anthracycline acylhydrazone derivatives are useful in the preparation of the conjugates of this invention. The acid-sensitive hydrazone bond of the conjugates of this invention allows the release of free anthracycline from the conjugates in the acidic external or internal environment of the target cell.