Although an association between LA activity and acquired hypoprothrombinemia has been described many years earlier,1, 5it was not until the early 1980s that the plasma of a patient with the acquired LA-HPS was shown to contain non-neutralizing antiprothrombin antibodies capable of binding to prothrombin in solution thus resulting in rapid clearance of the prothrombin–antibody complex.20The very low inhibitory activity found on factor II underlies the presence of mainly non-neutralizing antibodies. Here, F2 is linked to Hermansky-Pudlak syndrome.