F ibrinogen and the cold-insoluble globulin of plasma (CIg) are the main protein components of the heparin-precipitable fraction of normal plasma. The interactions among these proteins and heparin were examined. Heparin formed a cold-precipitable complex with purified CIg or with mixtures of CIg and fibrinogen but not with purified fibrinogen alone. Cryoprecipitation was augmented by addition of Ca⁺⁺ or by selection of optimal heparin levels; it was reduced or even abolished by raising the ionic strength or pH or both, or by raising the heparin concentration above that for maximum precipitation of CIg. Fibrinogen reduced the threshold for heparin-induced CIg cryoprecipitation and, by coprecipitating with heparin and CIg, increased the amount of precipitate that formed. In contrast to the heparin-precipitable fraction of normal plasma which contained both fibrinogen and CIg, that from a patient with congenital afibrinogenemia contained CIg but lacked fibrinogen. Normal plasma depleted of CIg by immunoabsorption failed to form a heparin-induced cryoprecipitate. Thus, CIg is essential for heparin-induced cryoprecipitation to occur. Fibrinogen, as assessed by chromatographic experiments with heparin-Sepharose columns, had a considerably lower binding affinity for heparin than did CIg, suggesting that it participates in precipitate formation mainly, if not entirely, by virtue of its affinity for CIg. The region of the fibrinogen molecule accounting for its precipitation with CIg appears to be localized in the carboxy-terminal segment of the Aα-chain; fibrinogen subfractions lacking this region failed to augment cryoprecipitation of heparin-CIg mixtures and, even though such species were present in normal plasma, they failed to coprecipitate in the heparin-induced complex.