FVIII activates factors that provide nonspecific costimulation. FVIII^−/−/C57BL/6 mice received 5 weekly injections of 1 μg FVIII intraperitoneally (n = 6) or 1 μg OVA intraperitoneally (n = 6) or 1 μg OVA + 1 μg FVIII (n = 6). This experiment has been repeated twice, and representative results are reported here. (A) Average concentrations of anti-FVIII antibodies in mice treated with FVIII only (270 ± 76 μg/mL) are not significantly different from mice treated with OVA + FVIII (293 ± 98 μg/mL). Values from mice treated with OVA represent the background for this assay and have been subtracted. (B) T-cell proliferation in response to FVIII has been measured for all 3 treatment groups. Again, there is no difference between FVIII and OVA + FVIII, in terms of response to FVIII. (C) Serum samples have been assayed for antibodies specific for OVA. When mice are treated with OVA + FVIII, they form measurable antibody responses to OVA (30.2 ± 7 μg/mL) that are significantly increased over those treated with only OVA (5.8 ± 2 μg/mL). Background values from mice treated with FVIII have been subtracted (**P = .004). (D) In addition, mice treated with OVA + FVIII have a measured T-cell response to OVA, whereas the other groups do not (**P = .009).