Various conditions were evaluated and modified to improve the sensitivity of the serum neutralization (SN) test for detecting antibody in pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV). Higher SN titers were consistently obtained by the addition of 20% fresh swine serum to the virus diluent and by the use of a permissive cell clone (MARC-145) derived from the MA-104 cell line. Test sera used to assess the SN test were obtained from 2 groups of 3-week-old pigs infected intranasally with PRRSV (MN-1b). Using the modified method, SN antibody was first detected 9-11 days postinoculation (PI), with a peak evident at 11-21 days PI. The antibody subsequently declined, and a second peak was observed between 41 and 45 days PI. The first antibody peak was not observed and the SN antibody was only detectable between 32 and 41 days PI when the test was done with 20% heated swine serum or without supplemental swine serum. The SN antibody during 2-3 weeks PI was found to be sensitive to 2-mercaptoethanol or anti-swine IgM treatment. The SN antibody titers were high when homologous PRRSV isolate was used in the test but were markedly low for heterologous PRRSV isolates. No difference in antibody titers was observed when homologous and heterologous PRRSV isolates were tested by indirect fluorescent antibody assay. These results indicate that the modified SN method is useful in detecting earlier and higher PRRSV antibody and that it can differentiate among PRRSV isolates.