Preparations of human plasma rich in antihemophilic factor (AHF, factor VIII) correct the coagulative defect of classic hemophilic plasma, form precipitates with specific heterologous antiserum, and support aggregation of platelets by ristacetin and retention of platelets by columns of glass beads. Whether these various properties can all be attributed to a single molecular species is disputed. Antiserums were prepared in rabbits to partially purified AHF and to high molecular weight (MW) and low MW fragments separated by gel filtration through columns of agarose in the presence of 0.25 M calcium chloride. Antiserums to AHF and to its high or low MW fragments all inactivated procoagulant AHF in plasma or in preparations of AHF. In contrast, antiserums to AHF and its low MW fragment inactivated procoagulant AHF in the low MW fragment, while that against the high MW fragment lacked this property. Thus, the low MW fragment appeared to have some antigenic sites not present or accessible to the antiserum against the high MW fragment. In agreement with this, the low MW fragment did not block antiserum against the high MW fragment as tested by the capacity of this antiserum to inactivate functional AHF in plasma. These immunologic studies support the view that the various properties of preparations of human AHF are attributes of a single molecular species.