Radioimmunoassays of human high molecular weight kininogen (HMWK) and low molecular weight kininogen (LMWK) were developed using antibodies directed against the light and the heavy chains of kallikrein-cleaved HMWK. With the anti-light chain antibodies, the radioimmunoassay was specific for HMWK with a detection limit of 0.4 ng. The anti-heavy chain antibodies were used to quantify the concentration of total kininogen antigens. In four different plasmas with a congenital deficiency in HMWK procoagulant activity, there was no detectable antigen in two cases and trace amounts, less than 1 micrograms/ml in the other plasmas (normal concentration: 72 +/- 6 micrograms/ml). In the absence of HMWK, the radioimmunoassay performed with the anti-heavy chain antibodies was specific for LMWK. The amount of LMWK was different in each of these patients' plasmas, ranging from no detectable antigen, i.e. less than 0.15 micrograms/ml, to a normal content. Antigens immunologically indistinguishable from plasma kininogens were detected in lysates of five times washed platelets. HMWK antigen concentration was 3.17 +/- 0.87 micrograms per 10(11) platelets (mean value in 11 donors). LMWK was also present in platelet lysates and the relative concentration versus HMWK was the same as in plasma.