The frequencies of circulating donor-reactive cytotoxic lymphocyte precursors (CLP) and Il-2-producing helper lymphocyte precursors (HLP) were determined by limiting dilution analysis in 19 kidney allograft recipients before and at various intervals (up to 2 years) after transplantation. A significant, selective, and stable reduction of the frequencies of donor-reactive (but not of third party-reactive) CLP and/or HLP was observed in some patients beginning 3 to 6 months after transplantation. One patient developed reduced frequency of CLP only, 3 patients reduced frequencies of HLP only, and 2 patients reduced frequencies of both CLP and HLP. The selective reduction of donor-reactive CLP and/or HLP frequencies ranged from 5-25-fold when compared with the pretransplantation level and was associated with stable graft function. These data indicate that functional deletion of circulating donor-reactive T cells can occur at the level of cytotoxic T lymphocytes, Il-2-producing helper T lymphocytes, or both. Implications of these findings for the individualization of immunosuppressive regimens will be discussed.