Most recently, we adapted a pharmacological inhibitor assay for the measurement of the inhibitory capacity of neutralizing anti‐FXIII‐A antibody by determining the antibody concentration required for 50% inhibition of FXIII (IC50). In this assay the combined inhibition of FXIII activation and FXIIIa activity is measured. Various concentrations of the patient's IgG or normal IgG are pre‐incubated with plasma FXIII, then FXIII is activated by thrombin and Ca2+ and FXIII activity is measured. Normal IgG had no effect on FXIII activity, and the IC50 value for the patient's IgG could be calculated (an IC50 tool kit is useful for the calculation; www.ic50.tk). A typical IC50 curve 23 is shown in Fig. 2(B). This assay was used in two further studies for the quantification of neutralizing anti‐FXIII‐A antibodies 24, 25. The advantage of IC50 determination over the Bethesda‐Nijmegen assay is the exact information on the inhibitory capacity of the antibody. Its disadvantage is the requirement of purified FXIII and IgG.In rare cases the anti‐FXIII antibody did not inhibit FXIII activity measured by an amine incorporation assay, but it caused impaired cross‐linking of fibrin chains. These type III antibodies are detected by monitoring the cross‐linking of fibrin γ‐ and α‐chains using SDS PAGE analysis of the fibrin clot 17, 18, 26. The inhibition of fibrin chain cross‐linking might also be a result of an antibody directed against fibrin(ogen) and not against FXIII 18.