The production of anti-tetanus toxoid (TT) antibody and anti-anti-TT (auto-anti-Id) antibody has been measured in three young and three old persons. Both antibodies were measured using ELISA. We found that: (i) the anti-TT response of old subjects was lower than that of young subjects, (ii) serum auto-anti-Id antibody concentration was higher in old compared with young humans before boosting with TT, and (iii) anti-TT antibodies from different humans shared Id cross-reactivity when tested with a rabbit anti-Id antibody. Thus, the serum anti-TT antibody response to boosting was inversely correlated with the serum level of auto-anti-Id. This conclusion is consistent with the view that the higher level of auto-anti-antibody in older subjects contributes to their impaired antibody response to TT.