Previous studies using monoclonal antibodies (mAbs) have revealed antigenic variation among UK isolates of porcine reproductive and respiratory syndrome viruses (PRRSV) and the use of in vitro translation products has shown that this variation lies in the protein encoded by open reading frame (ORF) 3. This protein has been shown to be present in purified virion preparations, suggesting that it is a structural protein. The original objective was to investigate the degree of variation of ORF3 among a number of UK isolates of different mAb reactivity and diverse chronology by sequencing and to correlate this with the mAb reactivity, in an attempt to define conserved and variable antigenic sites. A number of PRRSV isolates, from different outbreaks in the UK between 1991 and 1994, were propagated in pig alveolar macrophages and RNA extracted. The ORF3 and ORF7 regions of the individual viruses were amplified by the polymerase chain reaction (PCR) and their sequences were determined using internal sense and antisense primers. A number of differences among the sequences were noted within specific regions of the ORF3, with a hypervariable area detected at the carboxyterminal end, in the area of overlap with ORF4. With the most divergent isolate, 9.5% of the 84 translated amino acids encoded by the area of overlap were different from Lelystad isolate, translating the sequence in both reading frames. In view of consistent changes elsewhere in the ORF that suggest a common ancestry among the isolates studied, we conclude that this region may be subject to rapid change in comparison to other regions studied, and therefore may be an area subjected to immunoselective pressure.