T-cell co-stimulation delivered by the molecules B7-1 or B7-2 through CD28 has a positive effect on T-cell activation 1, whereas engagement of cytotoxic T-lymphocyte antigen 4 (CTLA-4) by these molecules inhibits activation 2. In vivo administration to mice of blocking monoclonal antibodies or Fab fragments against CTLA-4 can augment antigen-specific T-cell responses 3, 4 and, thus, therapy with monoclonal antibody against CTLA-4 has potential applications for tumor therapy and enhancement of vaccine immunization 5, 6, 7. The effects of B7-1 and B7-2 co-stimulation through CD28 depend on the strength of the signal delivered through the T-cell receptor (TCR) 8, 9 and the activation state of T cells during activation 10, 11. Thus, we sought to determine whether these factors similarly influence the effect of B7-mediated signals delivered through CTLA-4 during T-cell activation. Using freshly isolated human T cells and Fab fragments of a monoclonal antibody against CTLA-4, we demonstrate here that CTLA-4 blockade can enhance or inhibit the clonal expansion of different T cells that respond to the same antigen, depending on both the T-cell activation state and the strength of the T-cell receptor signal delivered during T-cell stimulation. Thus, for whole T-cell populations, blocking a negative signal may paradoxically inhibit immune responses. These results provide a theoretical framework for clinical trials in which co-stimulatory signals are manipulated in an attempt to modulate the immune response in human disease.