High molecular weight kininogen (HMWK) and low molecular weight kininogen (LMWK) in human plasma could be rapidly (36 min.) separated on a DEAE-Sepharose Fast Flow column (1.0 x 5 cm and 0.50 ml plasma) by applying a NaCl step gradient. Quantification was then carried out by the Laurell rocket method with an antiserum raised against HMWK. Standard preparations for the assays were (I) crude HMWK and (II) crude LMWK prepared by the one-step procedure mentioned. In disc PAGE (8% with 0.1% SDS) immunoblot showed two main bands in I, migrating to apparent mol.wts. of 180,000 and 120,000. The 180,000 band predominated in native plasma. Purified HMWK (spec.act. 14 micrograms bradykinin/A 280) yielded in addition a band corresponding to a mol.wt. of 100,000. Immunoblot of II showed one broad zone over the mol.wt. range 65-70,000. The average assay values obtained in human plasma specimens from 10 males were 85 micrograms/ml for HMWK (range 65-130) and 174 micrograms/ml for LMWK (range 164-183). HMWK occasionally lost immunoreactivity during purification without a corresponding loss of kinin. Such a loss of immunoreactivity seemed to run parallel with a reduced release of kinin induced by hog pancreas kallikrein.