[en] Antisera were raised in rabbits against immunosorbent-purified F(ab')₂ fragments of IgG antitetanus toxoid (TT) antibodies obtained from three different donors. The antisera were rendered idiotype specific by absorption with insolubilized TT-nontreactive F(ab')₂. The resulting anti-idiotypic antisera precipitated more than 85% of ¹²⁵I-radiolabeled F(ab')₂ anti-TT and were shown to be individual specific in that they reacted only with the immunizing F(ab')₂ anti-TT and not with F(ab')₂ anti-TT derived from other donors. Anti-idiotypic antiserum inhibited binding of ¹²⁵I-TT but not of ¹²⁵I-DT to IgG derived from the donor of the immunizing F(ab')₂ anti-TT, but did not inhibit ¹²⁵I-TT binding to IgG derived from other donors. Finally, binding of ¹²⁵I-F(ab')₂ anti-TT to the anti-idiotypic antiserum was completely inhibited by IgG derived from the same donor but only partially inhibited by a great excess of TT antigen, suggesting that the anti-idiotypic antiserum is recognizing nonantigen-binding idiotypic determinants on F(ab')₂ anti-TT. The data presented demonstrate that anti-idiotypic heteroantisera can be successfully raised against human antibodies to TT, indicate minimal cross-reactivity of idiotypic determinants between unrelated individuals, and suggest the presence of nonantigen binding-idiotypic determinants on the antibody molecule