Split products of fibrinogen and fibrin are found In the sera of patients with def ibrination syndrome and/or fibrinolysis. They may result from spontaneous (primary) fibrinolysls or secondary fibrinolysis of Intravascular fibrin deposits. The split products can be detected by several immunologic methods. Both immunodiffusion and immunoelectrophoresis in agar gel show abnormal bands In high-titer pathologic serum samples (usually more than 12 [mu]g/ml). One of the lines present on Immunodiffusion is closer to the point of application than the other. The position of the closer band might result from the presence of small amounts of fibrinogen-sized molecules or from moderate amounts of partially polymerized or digested fibrin. A precipitln test In a capillary tube offers a simple and sensitive method for demonstrating split products; immediate precipitin occurs with high-titer products but lesser amounts may require up to 18 hr. Incubation. The Fi test, agglutination of antibody-coated latex particles, is simple, rapid, moderately sensitive, and commercially available but sometimes yields false-positive results. The precipitin or Fi test on thrombin-treated blood, plasma, or serum may be positive when split products are present in high titer, can be read immediately, and thus provides a rapid bedside test. Neither the precipitin nor the Fi test is as sensitive as the tanned, red-cell hemagglutination inhibition immunoassay (TRCHII) for the quantitation of fibrinogen and its split products. Because defective and incomplete coagulation may coexist with fibrinolysis in these clinical syndromes, an excess of thrombin must be added to remove thrombin-clottable fibrinogen and establish the presence of nonclottable split products. Up to 768 [mu]/ml split products were detected in serum from patients with reduced fibrinogen with associated primary fibrinolysis (idiopathic, hepatic disease), Induced fibrinolysis (streptokinae, urokinase) or in defibrination syndrome with secondary fibrinolysis (metastatic cancer, abruptio placentae, diffuse allergic vasculitis). In other patients with secondary fiorinolysis, up to 96 [mu]g/ml were occasionally encountered during and following obstetrical delivery of normal or dead fetus, in pulmonary embolism, myocardial infarction, and rheumatoid arthritis. The actual quantity of split products was of greatest value in assessing clinical progress. Heparin therapy in patients with defibrination syndrome was associated with a rise in plasma fibrinogen and a fall in the concentration of split products. The data indicate that trace amounts are found both in primary and secondary fibrinolysis, which are relatively common disorders.