The immunosuppressive effect of OKT3 depends upon both T cell depletion and antigenic modulation of CD3 complex. To establish the effect of low doses of OKT3 on peripheral T lymphocytes, we analyzed 47 kidney transplant recipients receiving OKT3 for the first time. OKT3 was used as rescue therapy in 39 patients and as part of induction protocols in 8. The mean age of patients was 39+/-10 years, 30 were females and 9 were re-transplants. Half of them (51.1%) received kidney from cadaver donors. Among those receiving OKT3 as rescue therapy, 82% recovered graft function, including patients with severe BANFF-graded rejections. After the first dose of OKT3, it a pronounced T cell depletion was observed followed by an increase in CD4 and CD8 expression in CD3 negative T cells, supporting the idea that T cell modulation was present. In conclusion, low dose OKT3 was effective in treating severe allograft rejection by inducing a sustained TCR/CD3 down modulation without long-lasting T cell depletion.