A panel of poliovirus-specific murine CD4+ T-cell clones has been established from both BALB/c (H-2d) and CBA (H-2k) mice immunized with Sabin vaccine strains of poliovirus serotype 1, 2, or 3. T-cell clones were found to be either serotype specific or cross-reactive between two or all three serotypes. Specificity analysis against purified poliovirus proteins demonstrated that T-cell clones recognized determinants on the surface capsid proteins VP1, VP2, and VP3 and the internal capsid protein VP4. Panels of overlapping synthetic peptides were used to identify eight distinct T-cell epitopes. One type 3-specific T-cell clone recognized an epitope within amino acids 257 and 264 of VP1. Three T-cell epitopes corresponding to residues 14 to 28, 189 to 203, and 196 to 210 were identified on VP3 of poliovirus type 2. The remaining four T-cell epitopes were mapped to an immunodominant region of VP4, encompassed within residues 6 and 35 and recognized by both H-2Z and H-2k mice. The epitopes on VP4 were conserved between serotypes, and this may account for the predominantly cross-reactive poliovirus-specific T-cell response observed with polyclonal T-cell populations. In contrast, T-cell clones that recognize epitopes on VP1 or VP3 were largely serotype specific; single or multiple amino acid substitutions were found to be critical for T-cell recognition.