SummaryIn this study, we employed Particle Concentration Fluorescence Immunoassay (PCFIA), for directly measuring both high and low molecular weight kininogens (HK and LK) in human plasma. In 38 normal donors, the mean values for plasma kininogens were 93 mg/ml +/- 19 SD, 82 mg/ml +/- 12 SD, and 175 mg/ml +/- 29 SD, respectively for LK, HK, and TotK (the sum of LK and HK detected by their common heavy chains). Plasma completely deficient in HK and LK was unreactive (<0.25 mg/ml) in all 3 assays whereas plasma from a patient with Fitzgerald Trait had an HK value of 11 mg/ml, an LK value of 36 mg/ml and a TotK value of 59 mg/ml. The reagents can be prepared in advance and all three kininogen determinations can be performed, using the same diluted sample, on 24 plasma samples, in triplicate, or 40 plasma samples, in duplicate, in less than 1 h. By performing all 3 kininogen determinations, it is possible to differentiate cleaved from intact kininogens. This technique will facilitate the widespread screening of kininogen levels in biological fluids of normal humans as well as of patients with various diseases.