Factor VIII procoagulant activity (F VIII:C) and factor VIII related antigen (F VIII R: Ag) were investigated in 35 patients with Argentine hemorrhagic fever. Since the results obtained in the three clinical forms of the disease were not significantly different, they were tabulated altogether. F VIII:C was low in early stages of the disease but increased progressively in later days (days 5-6: 0.54 +/- 0.10 I. U/ml; days 13-14: 0.95 +/- 0.13 I.U./ml). In contrast, the levels of F VIII R: Ag were high all along the disease and they returned to normal values during the convalescence period (days 5-6; 2.58 +/- 0.54 I.U./ml; day 30: 1.30 +/- 0.14 I.U./ml). The levels of F VIII R: ag were similar in samples drawn before (11 cases) or after (10 cases) the treatment with immune plasma infusion. Plasma samples from 12 patients were studied by two-dimensional immunoelectrophoresis. The only abnormality found was increased height of the immune precipitation arc.