Antibodies against T cells are widely used as immunosuppressive agents in clinical therapy. As effector functions of chimeric or humanized anti-T cell antibodies cannot be predicted in vitro, we compared T cell-depleting effects of human isotypes in vivo with their immunosuppressive consequences in a mouse BMT model. This system is based on chimeric antibodies with a mouse pan T cell specificity and human constant regions. To secure optimal immunosuppression, the specificity for Thy-1.2—one of the best-characterized T cell antigens—was selected, as Thy-1.2-specific antibodies prevent graft-versus-host disease in fully mismatched mice. Chimeric mouse anti-Thy-1.2 antibody with the human IgG1 Fc part was found to be equally effective in preventing graft-versus-host disease mortality as the highly protective anti-Thy-1.2 mouse IgG2a isotype, while human IgG3 was far less effective. This was not predictable by measuring the degree of T cell depletion in peripheral blood. T cell depletion in lymph nodes, however, exactly reflected the results obtained in the BMT system. In addition, this system offers the advantage of assessing the influence of reduced antigen density by using heterozygous Thy-1.2 mice.