F glycoprotein has been identified as an important target structure in the immunological response following respiratory syncytial virus (RSV) infection. Two sequential B epitopes corresponding to amino acids 200 to 225 and 255 to 278 have already been defined with anti-RSV rabbit serum. The T helper response to peptides which belong to these sequences was investigated in this study. Proliferative T-cell responses to these peptides were analysed in BALB/c mice (H-2d) and others strains: SJL (H-2s), C3H/He (H-2k), B10.BR (H-2k) and C57BL/6 (H-2b). By using various strategies, two T-cell epitopes were identified in the amino acid 200 to 225 and 255 to 278 regions, close to a neutralizing epitope. The T-cell responses to these peptides were H-2- restricted. In addition, the peptide 255-278 was able to stimulate T cells that responded to a subsequent immunization with F glycoprotein in vitro.