Anticoagulants in the form of heparin, dipyridimole, steroids, prostaglandin E1, Macrodex, and antithrombin III were administered in separate experiments prior to endotoxin infusion in the dog. The pattern of disseminated intravascular coagulation (DIC) developed consistently when endotoxin alone was administered. Heparin dosages from 1 to 10 mg/kg did not influence the appearance of thrombocytopenia but effectively eliminated the decrease in fibrinogen levels ordinarily found. Antithrombin III (AT III), obtained from the National Red Cross, administered in a dose designed to provide a doubling of the circulating AT III, reduced the fibrinogen utilization to a similar degree as heparin without affecting the platelet loss. Dipyridimole, as administered, was ineffective in this model, and did not alter the development of thrombocytopenia or the hypofibrinogenemia. Steroids, Macrodex, and prostaglandin E1 had minimal effect on the coagulopathy. Our finding would suggest that the endotoxin effect on dog platelets id direct, and not mediated by thrombin, and that the role of heparin in the clinical management of DIC should be considered only in instances in which renal complications exist.