The development of FIX-specific antibodies after identical immunizations with rhFIX in the presence of adjuvant. All of these mice received 2 immunizations of rhFIX (BeneFIX) with adjuvant. All FIX^null mice and 3 of 6 BMT-L mice developed FIX antibodies. None of the 2bF9-L, 2bF9-H, or BMT-H mice developed antibodies. These results indicate that, at some threshold level of expression, 2bF9 may induce immune tolerance in FIX^null mice.