I have developed a new type ELISA using synthetic peptides as antigen for the detection of antibodies to the nucleocapsid protein of porcine reproductive and respiratory syndrome virus (PRRSV). This ELISA has several advantage over the presently available commercial ELISA (HerdChek) used commonly in the diagnosis of PRRSV infections of pigs, This new ELISA also has detected Abs in pigs of herds which are thought to be free of PRRSV since the pig sera were negative in the commercial ELISA and the herds were free of clinical symptoms. These results suggest that PRRSV infections may be more widespread than presently recognized probably involving PRRSV strains that cause only asymptomatic infections. The prevalence of such strains would impede eradication of PRRS and could be the source of new virulent viral strains.