The salient abnormalities of blood coagulation found in the acute phase of Argentine hemorrhagic fever (AHF) were thrombocytopenia, prolonged partial thromboplastin time activated with kaolin, low factor VIII:C activity concurrent with high levels of von Willebrand factor, and increased values of factor V. No evidence of disseminated intravascular coagulation (DIC) was observed. Therefore, the hemostatic abnormalities detected in patients with AHF could not be attributed to DIC. There was no correlation between severity of the disease and occurrence of impairment of coagulation. Complement activation was observed during the acute phase of AHF. There was reduction of total complement and C2 activity. Antigenic levels of C1q, C3, and C5 were low; level of C4 antigen was high. Degradation products of C3 and B were demonstrated before day 11. Experimental models of AHF were developed (guinea pigs, Callitrix jacchus). These models may be useful, but they reproduced only some of the features of blood coagulation and complement abnormalities described in human AHF.