The presence of high human rFIX protein concentrations during FVIII treatment results in reduced anti-FVIII antibodies. Comparison of (A) FVIII inhibitors, (B) anti-FVIII antibodies, and (C) anti-human FIX antibodies in hemophilic mice after 4 treatments of either 20 μg human rFIX, 3 μg human rFIX plus 2 IU rFVIII, 10 μg human rFIX plus 2 IU rFVIII, 20 μg human rFIX plus 2 IU rFVIII, or 2 IU rFVIII. Mice were sampled 1 week after the fourth treatment. Horizontal and error bars represent the mean and SEM, respectively, for 7 to 10 mice.