Summary
Variants of haemophilia B were examined by antibody-neutralization technique and by
the ox brain thromboplastin time method. On the basis of the presence or absence of
anomalous factor IX and of prothrombin time, four variants of haemophilia B were discerned.
It is shown that the prolongation of prothrombin time, measured in our cases, cannot
be attributed to an inhibitor in the patients’ plasma.