The murine MAb T10B9 is an IgM, Kappa antibody which binds preferentially to the an epitope on αβ TCR positive human T-cells. T10B9 modulates CD3 and TCR expression, inhibits antigen and mitogen induced proliferation, and prevents the induction of cytokines in αβ T-cells. When added to lymphocytes in the presence of complement, it induces T-cell lysis. T10B9 has been used in clinical investigations for ex vivo T-cell depletion in human bone marrow transplantations and for treatment of acute rejection of solid organ transplants. Efforts in our laboratory have involved the generation and expression of recombinant forms of this molecule. Data on the biochemical and biological properties of chimeric and humanized versions of T10B9 will be presented.