These studies have been directed at evaluating the role played by proteolysis (fibrinogenolysis) in vivo in prolonging the thrombin time of human umbilical cord (“fetal”) fibrinogen. The aggregation rate of cord fibrin compared with that from adult plasma is always delayed when the reaction is carried out under conditions of relatively high ionic strength (e.g., 0.29); this difference is not apparent at relatively low ionic strength (e.g., 0.09). In addition, as assessed by turbidimetric techniques, the maximum absorbance attained by cord fibrin is considerably less than that attained by adult fibrin. Coagulable fibrinogen catabolites (i.e., fraction I-5) are present in cord plasma and, like their counterparts from adult plasma, lack various portions of the COOH-terminal region of the A alpha chain. However, their presence in plasma does not explain the behavioral differences between cord and adult fibrin. Moreover, differences revealed by turbidimetric comparison of cord and adult fibrin from plasma fraction I-2 persist in fibrin from fraction I-5; it therefore appears that the COOH-terminal region of the A alpha chain does not contain the structure(s) accounting for the unique behavior of “fetal” fibrinogen.