A significant CD4+ T cell response against the transmembrane (M) protein can be detected in the spleens of C57Bl/6 mice infected intraperitoneally with a sublethal injection of the neurotropic JHM strain of mouse hepatitis virus (MHV-JHM), but not in those of mice with the chronic demyelinating encephalomyelitis caused by this virus. With the ultimate goal of determining the role of the M-specific response in the pathogenesis of MHV-JHM-induced neurological diseases, CD4+ T cell epitopes within the M protein were identified using vaccinia virus recombinants expressing truncated forms of the protein and peptides spanning most of the M protein in cell proliferation assays. Peptides covering residues 128-147 contain at least one CD4+ T cell epitope for MHV-JHM. Within this region is a sequence (residues 135-143) which matches the recently described MHC class II I-Ab binding motif. Delineation of this epitope should facilitate analysis of the role of the M-specific CD4+ T cell response in the development of acute and chronic neurological infections caused by MHV-JHM.