Some of the structural proteins of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) carry major epitopes involved in virus neutralization and are essential for the induction of protective humoral responses and the development of an effective vaccine. Rabbit antisera were prepared using full-length N and M proteins and eight expressed fragments covering the S protein. Antisera to S and M proteins were found to have different neutralizing titres towards SARS-CoV infection in vivo, ranging from 1:35 to 1:128. Antiserum to the N protein did not contain neutralizing antibodies. Epitopes inducing protective humoral responses to virus infection were located mainly in the M protein and a region spanning residues 13-877 of the S protein. The neutralizing ability of antisera directed against the expressed structural proteins was greater than that of convalescent patient antisera, confirming that, as immunogens, the former induce strong, SARS-CoV-specific neutralizing antibody responses. The in vitro neutralization assay has important implications for the design of an effective, protein-based vaccine preventing SARS-CoV infection.