Abstract A rat monoclonal antibody (MoAb), termed KBA, against mouse lymphokine-activated killer (LAK) cells recognizes a LAK cell surface molecule termed LAA responsible for the binding between LAK and target cells. In order to identify a target molecule of LAK cells, we prepared anti-KBA idiotype antibodies (anti-KBA-Id) from rabbit anti-KBA sera. Immunoglobulins were separated by ammonium sulfate precipitation followed by sequential affinity column chromatographies using Affi-gel coupled with rat MoAbs other than KBA and KBA-coupled gel. An immunoglobulin(s) in a KBA-gel-bound fraction showed the selective reactivity to KBA, comprising anti-KBA-Id character. This anti-KBA-Id inhibited the binding of KBA to LAK. Moreover, it bound with a portion of mouse leukemia cells sensitive to LAK cells, but not with normal mouse cells, and inhibited the binding of LAK cells to a target leukemia. These findings indicate that the anti-KBA-Id contain anti-Id which possess a three-dimensional structure that mimics a mirror image of the antigen (LAA)-combining site in KBA or the structure of LAA. The antigen reactive with anti-KBA-Id was characterized as a glycoprotein.