We studied the coagulation and complement systems during Rocky Mountain spotted fever in Macaca mulatta experimentally infected with Rickettsia rickettsii. Ninety-one percent of monkeys infected intravenously with a high dose (10(6) plaque-forming units [pfu]) and 56% of monkeys infected with low doses (10(-1)-10(2) pfu) of R. rickettsii died after two to four days of illness. With the onset of fever and rickettsemia, animals developed hyperfibrinogenemia, mild thrombocytopenia, prolonged prothrombin and activated thromboplastin times, and increased serum fibrin/fibrinogen degradation products (FDP). Rickettsemia, thrombocytopenia, and FDP were greater in fatally ill monkeys than in survivors. Hemolytic titers of the second and third components of complement were not depressed except in a single surviving monkey that developed peripheral gangrenous ecchymoses at a time when both rickettsemia and agglutinating antibody were present. Thus, although activation and consumption of complement may occur during Rocky Mountain spotted fever, the hemostatic disturbances in fulminant infections seem to be a direct effect of the infectious vasculitis.