The factor VIII-related antigen in a patient with von Willebrand's disease showed increased anodal mobility on two-dimensional immunoelectrophoresis. However, the antigen present in several of this patient's relatives showed two distinct peaks, one in the normal position and one similar to that of the patient. Very little of the patient's antigen could be recovered from ethanol precipitates or cryoprecipitates of whole plasma. Only the slower component could be recovered from the precipitates prepared from plasma of these family members showing two populations of antigen, the faster moving antigen remaining in the supernatant. This antigen of increased mobility corresponds very closely to that remaining in the supernatant of normal plasma, following similar fractionation procedures. It is suggested that the factor VIII-related antigen in some variant forms of von Willebrand's disease is a normal component of plasma, present in increased amounts.