Eleven epitopes were identified by murine monoclonal antibodies (MAbs) that represented the N, M, GP5 and GP3 proteins of the North American (NA) porcine reproductive and respiratory syndrome (PRRS) virus, KY 35 (NVSL 46907). Three discontinuous epitopes of the N and M proteins were designated EpORF7-Fd through Hd and EpORF6-Ad through Cd. Five continuous epitopes of the GP5 and GP3 proteins were designated EpORF5-A through C and EpORF3-A and B. The MAbs representing EpORF5-C and EpORF6-A and B had neutralizing activity. The MAbs representing the above epitopes, except EpORF7-Gd and Hd, expanded the virus marker system described in a previous study in which a panel of 69 NA viruses and the Lelystad virus were categorized into 5 antigenic groups, I15 through V15 based on the presence or absence of 5 continuous epitopes of the N protein. Antigenic groups I15 and II15, which represented 84.7 and 11.6% of all viruses tested, were categorized further into 9 and 4 subgroups, respectively. The remaining NA viruses and the Lelystad virus were distributed among 4 groups, one of which was represented by 2 subgroups. Significant (P<0.05) differences in sensitivity to neutralization of 28 viruses representing 6 antigenic groups by the 3 neutralizing MAbs suggested that sensitivity to neutralization may also be of value in categorizing PRRS viruses.