It is a monoclonal antibody to CD33 linked to a cytotoxic agent, calicheamicin. CD33 is expressed in most leukemic blast cells but is not found on normal hematopoietic stem cells. When given to patients in first relapse, 15% of patients achieve a complete remission. In the United States, it was approved by the FDA for use in patients over the age of 60 with relapsed AML who are not considered candidates for standard chemotherapy.

A notable complication of therapy with gemtuzumab is the increased risk of veno-occlusive disease in the absence of bone marrow transplantation. The onset of VOD can also be delayed and can occur at increased frequency following bone marrow transplantation.