Genetic, antigenic, and pathogenic variability is known to exist among porcine reproductive and respiratory syndrome (PRRS) viruses and has garnered great attention for diagnostics and disease control/prevention. A comparative serologic study was conducted on five field and two cell-attenuated viruses to determine if serologic responses to PRRS virus infection could be influenced by biotypic and/or genotypic differences of the viruses. The isolates used for the study varied in their virulence to pigs and in genomic sequences. Ten pigs were inoculated with each isolate (1x10(3) TCID(50)) via the intranasal route. All inoculated animals became viremic during the study period. Some animals inoculated with the attenuated viruses remained seronegative until the end of the study (42 days post-inoculation (PI)), but all of the animals inoculated with field viruses developed enzyme-linked immunosorbent assay (ELISA)- and indirect fluorescent antibody (IFA)-detectable antibodies, regardless of the virus strain used in the IFA assay. In contrast, a great degree of variation in the onset and level of serum-virus neutralization (SVN) antibody was observed by individual pigs and by each virus. The reactivity of SVN antibody was highly specific for homologous viruses. Cross-neutralization titers were better correlated with sequence homology of open-reading frames (ORFs) 4 and 5 among the viruses than any other structural genes. We conclude that the biotypic difference among PRRS viruses may affect the kinetic of humoral immune response in infected pigs. The IFA test may be used as a confirmatory test when a false-positive ELISA result is suspected or vise-a-versa, but SVN antibody titers are highly affected by antigenic variability.