Auto-anti-idiotypic antibodies (Aab-2s) were detected from pigs experimentally infected with porcine reproductive and respiratory syndrome virus (PRRSV). The Aab-2s were specific against the idiotypic antibodies (Ab-1s) to the envelope glycoprotein GP5 of PRRSV and were detected from serum samples collected between 21 and 98 days post-infection (DPI). Serological characterization indicated that the Aab-2s recognized the idiotype located within or near the antigen-combining sites of the anti-GP5 antibodies, which was shared by both mouse MAb anti-GP5 and swine polyclonal antibodies. The fact that the Aab-2 inhibited the anti-GP5 antibodies from binding to PRRSV and that they were detected at different time periods in pigs that cleared the infection prior to 98 DPI versus pigs in which virus was detected at 98 DPI suggests that Aab-2 antibodies may play a role in immunity to PRRSV infection.