Abstract The monoclonal anti-CD3 antibody is used as part of prophylaxis and also in treatment of rejection. In the present article we analyzed changes in different lymphocyte subpopulations after anti-CD3 treatment. T lymphocytes were decreased under anti-CD3 antibody administration, with a simultaneous increase in B lymphocytes but no changes in natural killer (NK)cells. No differences were found between patients administered anti-CD3 antibody (Ab) at 5 versus 2.5 mg/d. It is uncertain whether these changes may be implicated in the lack of response or in the prophylactic effects of anti-CD3 Ab.