We have previously defined major histocompatibility complex (MHC) class II-restricted T-cell epitopes from the carboxy-terminal region of group A streptococcal type 5 M protein. In this report, T-cell responses to one of these epitopes have been characterized in detail. T-cell clones from recombinant M5-immunized mice and popliteal lymph node cells from peptide-immunized mice were used to show that sM5[300-319] is recognized in the context of I-A alleles of four of nine independent MHC class II haplotypes: I-Ad, I-Af, I-Ak, and I-As. This epitope was also recognized by both helper (Th2) and inflammatory (Th1) subsets of murine T cells. The I-Ad-restricted epitope recognized by BALB/c mice was mapped to the 12-amino-acid peptide sM5[308-319] and was shown to provide helper function for an immunoglobulin G anti-peptide antibody response in BALB/c mice. Anti-peptide antibody was shown to be specific for M5[304-315] but failed to recognize intact rM5, suggesting that the conformation of the epitope differed between peptide and protein. However, the results demonstrate that overlapping epitopes can be the focus for both immunoglobulin G antibodies and the T cells which augment their production. Comparison of the available sequences for M proteins indicated that the T-cell epitope within M5[300-319] was highly conserved between M types and hence may elicit helper function for protective antibody responses to a wide range of M types. T-cell epitopes from conserved regions of M proteins which are recognized in the context of multiple MHC haplotypes have potential for the design of multivalent streptococcal vaccines.