Sequential lymph node biopsies were obtained prior to and following OKT3 administration in 10 organ transplant recipients to determine whether activation of human T cells occurs in vivo during OKT3 administration. Within 2 hr after injection, OKT3 can be detected coating LN T cells, and LN T cells also demonstrate enhanced proliferation in vitro in the presence of recombinant interleukin-2 (rIL-2). Interleukin-2 receptor (IL-2R) is expressed on post-OKT3 LN T cells within 48 hr following administration of OKT3. In addition, when placed in a mixed lymphocyte reaction, post-OKT3 LN cells also demonstrate enhanced proliferation. This is the first direct demonstration of in vivo activation of human T cells by OKT3. These data support the hypothesis that T lymphocyte activation and concomitant production of lymphokines are responsible for the side effects associated with OKT3 treatment. Immune activation and possible enhancement of anti-donor MHC alloreactivity may have significant implications for anti-CD3 and anti-TCR monoclonal antibody therapy in clinical organ transplantation and for enhancement of the immune response in cancer patients.