Of a total of 187 consecutive liver grafts in 149 pediatric recipients, 59 episodes of steroid-resistant, biopsy-proven rejection (32% of grafts) were treated with OKT3 monoclonal antibody. After 59 OKT3-treated episodes, liver function at the end of treatment was normal in 40%, improved in 35%, and unchanged in 24%. Of 21 partial responses, 12 episodes eventually resolved to yield an overall complete response rate of 59%. CD3-positive T-cells greater than 5% occurred during 61% of OKT3-treated rejection episodes and was associated with impaired efficacy of OKT3 (30% complete response rate). Six grafts were re-treated with OKT3 for rejection and in all CD3-positive T-cells could not be maintained less than 5%. Of the six grafts requiring repeat OKT3, five failed-retreatment and required retransplantation. OKT3 antibodies of low titer were found prior to a second use of OKT3 in 65% of episodes. Patients treated with OKT3 after failing more than two preceding steroid courses had a significantly increased chance of graft loss (57%; p = 0.01). We conclude that this group of pediatric patients appeared less responsive to OKT3 compared to other series combining pediatric and adult recipients, possibly due to a more vigorous immune response in the child.