The antibody response to low-dose wt fVIII and R372A/R1689A fVIII in fVIII^−/− mice. FVIII^−/− mice were injected with 6 weekly doses of wt fVIII (n = 25) or R372A/R1689A fVIII (n = 25) of 0.2 μg, followed by 2 additional doses of 0.5 μg. One week after the last dose, plasma was collected for measurement of (A) total anti-fVIII IgG by ELISA and (B) fVIII inhibitor titers by the Bethesda assay. The difference in ELISA titers and inhibitor titers between the 2 groups was significant (P = .03 and 0.02, Mann-Whitney test). (C) The binding of biotinylated ESH4, a mAb that recognizes the classic C2 domain epitope overlapping a VWF binding site, was measured in the absence (filled circles) and presence (open triangles) of a 1/24 dilution of a high-titer plasma from a mouse immunized with R372A/R1689A fVIII. *P < .05.