A low molecular weight (LMW) kininogen was isolated from pooled human serum by chromatography on DEAE-Sephadex A-50, CM-Sephadex C-50, Sephadex G-150, and Sephadex G-100. It was shown to be homogeneous by ultracentrifugation, polyacrylamide gel electrophoresis, and immunoelectrophoresis. The sedimentation coefficient, S020,W, of purified LMW kininogen was 3.85 s, and its molecular weight was determined to be 78,000 by Sephadex G-100 gel-filtration. The LMW kininogen contained 79.3% protein, 8.0% hexose, 3.9% hexosamine, and 4.9% sialic acid. In order to determine the immunochemical properties of LMW kininogen, specific antiserum was prepared in rabbits. The antigenic determinant of LMW kininogen was not related to the sialic acid and kinin moieties in the kininogen molecule, but could not be distinguished from that of high molecular weight (HMW) kininogen. In the quantitative single radial immunodiffusion test, a sialic acid-free LMW kininogen reacted to a greater extent with the antiserum than the native LMW kininogen. The kininogen level in human serum was estimated by single radial immunodiffusion. The antiserum cross-reacted with monkey serum, but not with sera from dogs, rats, and mice, horses, pigs, guinea pigs, oxen, and rabbits.