Humanized or chimeric anti-CD99 monoclonal antibodies are provided. The antibodies bind to and neutralize human CD99, and find use in various therapeutic methods. Embodiments of the invention include isolated antibodies and derivatives and fragments thereof, pharmaceutical formulations comprising one or more of the humanized or chimeric anti-CD99 monoclonal antibodies; and cell lines that produce these monoclonal antibodies. Also provided are amino acid sequences of the antibodies.