Summary. Hemophilia A patients treated with coagulation factor (F)VIII may develop an anti-FVIII immune response. Anti-FVIII antibodies may occur also in healthy subjects. To understand the extent to which an immune response to FVIII occurs in healthy subjects, we investigated the proliferative response of blood CD4 T cells from 90 blood donors to FVIII and to pools of overlapping synthetic peptides spanning the sequences of individual FVIII domains (A1±A3, C1±C2). Most subjects responded to FVIII and several FVIII domains. Men had stronger responses to FVIII than women, and older subjects than younger subjects. The domain-induced responses were weaker than the FVIII-induced responses, yet their intensity in individual subjects correlated with that of the response to FVIII. We examined whether Th1 and/or Th2 cells responded to FVIII in 68 subjects, by determining the CD4 T cells that secreted interferon-g (IFN-g) or interleukin (IL)-5 after stimulation with FVIII: 25 subjects had FVIII-speci®c IFN-g-secreting cells, and seven of them had also FVIIIspeci®c IL-5-secreting cells. None had only IL-5-secreting cells. Thus, a CD4 T cell response to FVIII, which ®rst involves Th1 cells, is common among subjects with a normal procoagulant function.