Porcine reproductive and respiratory syndrome (PRRS) is an economically significant disease of swine, caused by a small, enveloped RNA virus belonging to the family Arteriviridae. PRRS virus preferentially replicates in macrophages and is capable of establishing persistent infection. While the mechanism by which PRRS virus persists in infected animal is unknown, enhanced infection and replication of PRRS virus in the presence of virus-specific antibody has been demonstrated in vitro and in vivo. This phenomenon has been known as antibody-dependent enhancement (ADE) of virus infection in which virus-specific antibody facilitates the entry of virus into susceptible cells resulting in increased severity of the disease. ADE is also considered to be a significant obstacle to developing effective vaccines for many viruses for which ADE has been reported. In this study, the role of specific PRRS viral epitopes in ADE and/or virus neutralization (VN) was assessed in vitro, using 14 monoclonal antibodies (MAbs) to 4 PRRS viral proteins: 15kD nucleocapsid (N), 19kD matrix (M), 25kD envelope glycoprotein (GP5), and 45kD GP3, each of which represented a distinct epitope.One-way ADE and VN assays were performed using homologous PRRSV isolates in the presence or absence of each MAb. ADE activity was determined by detecting an increase of progeny virus yield in porcine alveolar macrophage cultures in the presence of individual MAbs. Neutralizing activity was determined by detecting a significant reduction or complete blocking of virus replication in MARC-145 cells in the presence of individual MAbs. The MAbs could be categorized into 3 groups: enhancing, neutralizing and neither. Neutralizing epitopes appeared to reside on the M, GP5, and GP3 proteins. ADE epitopes were associated with the N and GP5 proteins. Identification of the epitopes responsible for ADE and VN may provide the basis for developing efficacious second-generation vaccines for the control of PRRS virus.