A rather large proportion of the hemophilia B patients can be characterized as hemophilia B+ because of the presence in their plasma of a protein which is immunologically identical with human factor IX. In a group of 33 hemophilia B patients we found 14 cases of hemophilia B+ belonging to 11 independent pedigrees. The variant factor IX molecules of these families have been compared with respect to the following properties: 1) factor IX activity and its dependence on phospholipid concentration; 2) factor IX antigen; 3) prolongation of prothrombin time with an ox brain thromboplastin; 4) electrophoretic mobility; 5) Ca(+) binding capacity; 6) affinity for binding to heparin and 7) susceptibility of the factor IX antigen to contact-induced activation. In the study of these parameters the use of a precipitating antibody against highly purified human factor IX showed to be of great value. According to our criteria at least 7 different factor IX variants were present in the 11 families with hemophilia B+ studied. Because of this rather high heterogeneity a suitable nomenclature for subclassification of hemophilia B+ variants is proposed.