Cold-insoluble globulin is normally present in plasma and serum at concentrations of 27.52 +/- 4.60 and 23.46 +/- 5.18 mg/dl, respectively (means +/- SD). The concentration of CIg in blood samples was significantly decreased in DIC syndromes (14.69 +/- 6.55 mg/dl; p less than 0.001). A strong, positive correlation was found with AT-III (r = 0.68) and a less striking one with Plg. Although alpha 2-PI was shown to be significantly decreased in DIC syndromes (p less than 0.001), a weak, inverse correlation was found between CIg and alpha 2-PI (r = -0.29). Immunologically cross-reactive substances were found to be widely distributed in association with the cells and tissues of mesenchymal origin, including fibroblasts, adipose cells, smooth muscle cells, and basement membranes. The glomerular basement membrane was an exception and is currently believed to be of different origin. In the kidney, fluorescence was found in the mesangium. Cold-insoluble globulin is also present as a component of cryofibrinogen that forms a solid gel at low temperatures. Sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed that CIg in this fraction was rather homogeneous. Although closely migrating doublets were occasionally seen in the 440,000-dalton region on gels of unreduced samples, monomeric derivatives with a molecular weight of 220,000 or less, which have been claimed to occur in circulating plasma, were not observed. Thus, intact dimeric CIg appears to be the form of the molecule that complexes with fibrinogen. Cold-insoluble globulin is the fraction that was shown to exist as an independent entity from fibrinogen at an ambient temperature by immunoelectrophoresis and ultracentrifugation. However, very rapid formation of highly polymerized complexes in the sol phase at low temperatures was manifested by the finding of a sharp increase in light-scattering intensity using the technique of quasielastic light scattering. A control study on a mixture of normal CIg and fibrinogen disclosed no appreciable change in the temperature range between 37 and 8.5 degrees C. A comparative study on a mixture of cryofibrinogen-derived CIg and normal fibrinogen revealed an intermediate light-scattering pattern. After 2 hr at 8 degrees C, this mixture reached a state of equilibrium, where no further polymerization occurred. The secondary structures of normal and cryofibrinogen-derived CIg, determined by circular dichroism, showed no appreciable difference. A noteworthy finding was the almost complete absence of alpha-helices and a relatively high proportion of beta-structure in both forms of CIg. Amino termini of the fibrinogen moiety of cryofibrinogen were found to consist of alanine, tyrosine, and a small quantity of aspartic acid, consistent with the NH2 terminal moiety composition of normal fibrinogen but not of soluble fibrin monomer complex.