An antibody-drug conjugate (ADC) containing labetuzumab, a mildly reduced, anti-CEACAM5 humanized monoclonal antibody, conjugated to the potent topoisomerase I inhibitor SN-38, with antineoplastic activity. The monoclonal antibody moiety of labetuzumab govitecan selectively binds to carcinoembryonic cell adhesion molecule 5 (CEACAM5), which is abundantly expressed on the surface of a majority of solid tumors. Upon internalization and proteolytic cleavage, SN-38, the active metabolite of irinotecan, inhibits the activity of topoisomerase I in the tumor cells, eventually inhibiting both DNA replication and transcription and leading to tumor cell apoptosis.