A recombinant, humanized anti-CD33 monoclonal antibody attached to the cytotoxic antitumor antibiotic calicheamicin. In this conjugate, the antibody binds to and is internalized by tumor cells expressing CD33 antigen (a sialic acid-dependent glycoprotein commonly found on the surface of leukemic blasts), thereby delivering the attached calicheamicin to CD33-expressing tumor cells. Calicheamicin binds to the minor groove of DNA, causing double strand DNA breaks and resulting in inhibition of DNA synthesis. (NCI04)