Plasma samples from patients with inherited combined factor V and factor VIII deficiency were examined by immunological methods for the presence of factor V and factor VIII‐related antigens. A factor V‐related antigen was consistently demonstrated in all plasma samples by inhibitor neutralization assay using a non‐precipitating rabbit antibody. Factor VIII‐related antigens were detected by inhibitor neutralization using human antibody and by electroimmunoassay using a precipitating rabbit antibody. The amounts of factor V and factor VIII‐related antigens present in the patient's samples were similar to those found in normal human plasma. The findings confirm the presence of normal levels of factor VHI‐related antigen in the plasma of these patients and suggest that inactive antigenic determinants of procoagulant factor V and procoagulant factor VIII are also present. The results are consistent with the possibility that a common precursor of procoagulant factor V and factor VIII is defective in these patients.