We have established an ELISA for detecting thrombin cleavage of the FVIII light chain at Arg1689. The method used a coating alloantibody which recognized amino acid residues 2248-2312 in the C2 domain, together with a second monoclonal antibody, NMC-VIII/10, which recognized residues 1675-1684 in the amino-terminal region of the light chain. FVIII antigen (FVIII:Ag) was measured after treatment of plasma with various concentrations of thrombin. The FVIII:Ag of normal plasma was reduced in a dose-dependent manner by the thrombin, falling to 28% in the presence of 100 U/ml enzyme. The concentration of thrombin that achieved 50% reduction (IC50) was approximately 1.0 U/ml. The plasma of four haemophilia A positive (A+) and two haemophilia A reduced (AR) patients were analysed. The IC50 of all patients was more than 1.0 U/ml, indicating that thrombin cleavage of the FVIII light chain was defective. One haemophilia A+ plasma did not respond to thrombin in this ELISA system. The patient (TI) was a haemophiliac with FVIII coagulant activity of 0.04 U/ml and FVIII:Ag of 1.78 U/ml. In addition, immunoblotting of the purified FVIII from TI showed that thrombin cleavage of the 80 kilodalton (kD) light chain was impaired. The patient's DNA was amplified using the polymerase chain reaction with a set of synthetic oligonucleotide primers spanning amino acid residues 1646-1714. Sequence analysis of the amplified DNA fragments revealed a cytosine to thymine transition, converting an arginine 1689 to cysteine. This abnormal FVIII was designated as FVIII Hiroshima. Our ELISA system is a simple and useful method of evaluating the proteolytic cleavage by thrombin at Arg1689.