Abstract
Summary. A 67‐year‐old man developed a potent inhibitor of factor V shortly after undergoing blood transfusions and surgery. The inhibitor reacted in a time‐dependent manner with factor V and gave a non‐linear concentration graph. Studies with specific antisera indicated that it contained γ and μ heavy chains and both κ and λ light chains. The plasma from two patients with congenital factor‐V deficiency did not neutralize the inhibitor.