Previous studies have shown that treatment of mice in vivo with xenogeneic anti-idiotype produced against a monoclonal anti-H-2Kk antibody, 11-4.1, leads to the induction of molecules (Id') that inhibit the binding of anti-idiotype to idiotype. To investigate the nature of these Id' molecules, spleens from such anti-idiotype-treated mice were fused with the SP2/0 myeloma to produce monoclonal Id' antibodies. All four monoclonal Id' antibodies were found to react with goat and rabbit anti-11-4.1 in addition to the pig anti-idiotype used for their induction and one of the four, J1-8-1, reacted with syngeneic BALB/c anti-11-4.1. Partial amino acid sequences were determined for the heavy and light chains of these monoclonal antibodies. J1-8-1 heavy chain had an NH2-terminal amino acid sequence identical to that of 11-4.1 for the 39 NH2-terminal residues assigned, whereas its light chain and the heavy and light chains of the other Id' molecules differed markedly from those of the 11-4.1 antibody. Isolated heavy chains and light chains of J1-8-1 and 11-4.1 were reassociated in homologous and heterologous pairs. When J1-8-1 heavy chains and 11-4.1 light chains were mixed in equimolar concentrations, anti-H-2Kk reactivity was found at a level approximately 10% of that observed for reassociated 11-4.1 homologous heavy and light chains. The finding that in vivo anti-idiotype treatment can trigger Id' molecules structurally similar to the original idiotype has implications regarding the mechanism of induction of Id' molecules and the regulation of repertoire expression by idiotypic networks.