An RIA for human HMW kininogen, capable of detecting 150 pg of antigen, has been developed. Antibody to HMW kininogen was purified by immunoaffinity chromatography, and double-antibody precipitation was used to separate free and bound antigen. Of the LMW kininogens only one of the forms tested (B3.2) showed significant cross-reaction (2%). Bradykinin and human plasma kallikrein both showed no cross-reaction, and monkey HMW kininogen showed identity to the human antigen. Intraassay and interassay coefficients of variation were 2% and 1.5%, respectively. Recovery of HMW kininogen added to 6 plasmas was 97.7% +/- 1.8%. Assay of 17 normal plasmas gave a level of 90.8 +/- 2.5 micrograms/ml HMW kininogen (mean +/- S.E.M.). A bioassay of the samples, based on specific release of kinin by purified plasma kallikrein, yielded a level of 90.2 +/- 2.8 micrograms/ml HMW kininogen (r = 0.83, p less than 0.001). In neither assay was any significant sex difference observed. No evidence of any antigenic fragments was seen upon gel filtration of normal plasmas. RIA measurements were also performed on seven plasmas reportedly deficient in HMW kininogen. Williams, Dayton, San Francisco, and Flaujeac plasmas all showed no significant cross-reaction, whereas Fitzgerald, Reid and Detroit plasmas showed 1.0%, 2.5%, and 3.5% of normal antigenic levels, respectively. This sensitive, convenient method should facilitate studies on the role of the kallikrein-kinin system in health and disease.