Anti-bovine prothrombin antibodies in patients with antibodies to factor V. Plasma or sera samples from the patients were diluted into block buffer and incubated in microtiter plate wells coated with bovine prothrombin (for patient WH, a 1:100 dilution of the IgG fraction was used instead of plasma). Antibody binding to bovine prothrombin was detected with a peroxidase-conjugated goat anti-human IgG antibody. The IgG fractions were: H1 (▪); H3 (□); RS (⧫); RB (◊); WH (•); AR (○); EM (▴); RH (▵); H4 (); H5 (); MJ (); and BH ().