The only severe acute respiratory syndrome (SARS) vaccine currently being tested in clinical trial consists of inactivated severe acute respiratory syndrome-associate coronavirus (SARS-CoV). However, limited information is available about host immune responses induced by the inactivated SARS vaccine. In this study, we demonstrated that SARS-CoV inactivated by beta-propiolactone elicited high titers of antibodies in the immunized mice and rabbits that recognize the spike (S) protein, especially the receptor-binding domain (RBD) in the S1 region. The antisera from the immunized animals efficiently bound to the RBD and blocked binding of RBD to angiotensin-converting enzyme 2, the functional receptor oil the susceptible cells for SARS-CoV. With a sensitive and quantitative single-cycle infection assay using pseudovirus bearing the SARS-CoV S protein, we demonstrated that mouse and rabbit antisera significantly inhibited S protein-mediated virus entry with mean 50% inhibitory titers of 1:7393 and 1:2060, respectively.These data suggest that the RBD of S protein is a major neutralization determinant in the inactivated SARS vaccine which can induce potent neutralizing antibodies to block SARS-CoV entry. However, caution should be taken in using the inactivated SARS-CoV as a vaccine since it may also cause harmful immune and/or inflammatory responses. (C) 2004 Elsevier Inc. All rights reserved.