ELISA competition assays show that thrombin-modified factor XIII′ is the prime target of the patient’s antibody. Patient IgG (isolated on a Protein A column from serum drawn 9/12/91; 10 μg in 2% Blotto) was mixed with varying amounts of factor XIII (A₂B₂ or rA₂) or thrombin-treated factor XIII (A₂′B₂ or rA₂′) for 30 minutes at room temperature and the mixtures were then added to wells previously coated with 0.4 μg A₂B₂. After overnight incubation at room temperature, the human IgG bound to the immobilized antigen in the wells was detected with an alkaline phosphatase-conjugated antibody (γ-chain specific). For experimental details, see Materials and Methods.