Six patients with meningococcemia were studied; soluble circulating fibrin was measured with a technique based on the ability of plasma transglutaminase (factor XIII, the fibrin stabilizing enzyme) to incorporate 14 C glycine ethyl ester into fibrinogen after the alpha peptide of fibrinogen has been hydrolyzed by thrombin. Soluble circulating fibrin initially was found in all six patients, indicating active intravascular coagulation. All patients were treated with antibiotics and heparin. The disappearance of soluble circulating fibrin in two patients treated simultaneously with heparin and antibiotics was biphasic, with an initial rapid phase followed by a more gradual decline. A similar biphasic disappearance has been reported in experimental animals. In three patients treated with antibiotics before sampling and treatment with heparin, the initial rapid phase was not seen, suggesting intravascular coagulation had ceased prior to the time of sampling and heparin treatment. Although circulating fibrin was not detected in any patient after 15 hours of heparin therapy, two patients died. The half disappearance time of soluble circulating fibrin ranged from 8 to 15 hours with an average of 12 hours. The results indicate that the control of intravascular coagulation does not insure survival, and furthermore it is possible heparin may not be the critical factor in abolishing intravascular coagulation. However, a controlled study of heparin-treated and untreated patients will be necessary to adequately evaluate the effectiveness of heparin in patients with meningococcemia.