Human plasm contains at least two distinct kininogens, designated high Mr and low Mr kininogens, that differ in their Mr and susceptibility to different kallikreins. By limited proteolysis, plasma kallikrein cleaves high Mr kininogen to liberate kinin and gives a molecule containing two disulfide-linked polypeptide chains. Following reduction and alkylation of the two-chain molecule, the heavy chain and the light chain were isolated, and antisera were raised in goats against the alkylated heavy and light chains. The anti-heavy chain antiserum immunoprecipitated both high Mr and low Mr kininogen, whereas the anti-light chain antiserum was specific for high Mr kininogen. Thus, the heavy chain of kinin-free high Mr kininogen and low Mr kininogen extensively share identical immunologic determinants. In contrast, the immunologic determinants of the isolated light chain are unique to high Mr kininogen. Using immunochemical techniques, it was shown that high Mr kininogen or the light chain derived from it forms a complex either with prekallikrein or with kallikrein. Titrations of prekallikrein or kallikrein with increasing amounts of either high Mr kininogen or its alkylated light chain indicated that the complexes contain equimolar amounts of each molecule. These results show that a single site for prekallikrein or kallikrein binding to high Mr kininogen resides in the light chain region of the molecule.