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Porcine reproductive and respiratory syndrome (PRRS), an economically important swine disease, is caused by a previously unrecognized virus referred to as porcine reproductive and respiratory syndrome virus (PRRSV). In this study, PRRSV was isolated and characterized in a continuous cell line designated as ATCC CRL11171. PRRSV was found to be an enveloped, nonhemagglutinating, and spherical RNA virus ranging from 50 to 70 nm in diameter. The genome of PRRSV is a positive-stranded, polyadenylated RNA of approximately 15 kb. Six open reading frames (ORFs) were identified in the 3'-terminal viral genome. ORFs 2 to 4 are predicted to encode viral membrane-associated proteins, and ORFs 5 to 7 likely encode envelope, matrix and nucleocapsid proteins, respectively. A nested set of six or seven subgenomic mRNAs (sg mRNAs) was found in CRL11171 cells infected with different PRRSV isolates. These sg mRNAs were not packaged into virions. The leader-mRNA junction sequences of sg mRNAs 3, 4 and 4-1 contain a common six nucleotide sequence motif, T(G)TA(G/C)ACC. Extensive sequence variation exists between U.S. and European PRRSV isolates. The amino acid sequence identity in ORFs 2 to 7 between U. S. isolate VR2385 and European isolate LV is only about 60%. Sequence variation was also found among North American isolates. The amino acid sequence identity in ORFs 2 to 4 between seven North American isolates was 91-99%, 86-98%, 92-99% and 88-99%, respectively. However, ORFs 6 and 7 of all North American isolates were highly conserved. A low virulence U.S. isolate had highest sequence variation in ORFs 2 to 4 compared with that of other U.S. isolates. Phylogenetic analyses showed that PRRSV was closely related to lactate dehydrogenase-elevating virus (LDV) and distantly related to equine arteritis virus (EAV). The U.S. and the European isolates of PRRSV represent two different genotypes, and, within the major U.S. genotype, there are at least three minor genotypes. The results from th