Peptides P1 and P17 are HLA-DRB1*1501–restricted FVIII apitopes. T-cell hybridoma clones derived from PRCLT- or DNIMV-immunized HLA-DR2tg mice were cocultured with an Epstein-Barr virus–transformed human HLA-DRB1*1501–expressing B cell line (MGAR). MGAR cells were left untreated (fresh) or were fixed with 0.5% paraformaldehyde (fixed) to prevent antigen processing. Recognition of peptides P1, P17, PRCLT, and DNIMV or rhFVIII by 2 clones specific for PRCLT (A) and 2 clones specific for DNIMV (B) was addressed after 48 hours by analyzing IL-2 cytokine secretion in the supernatants by ELISA.