Fibroproliferative tumors, known as retroperitoneal fibromatosis (RF) are predominantly associated with type D simian retrovirus serotype 2 (SRV-2) infections in pigtailed macaques (Macaca nemestrina) at the University of Washington Regional Primate Research Center (UWRPRC). In this report we examined genetic changes that occurred in SRV-2/Washington (SRV-2/WA) from archived RF tissues and from fresh tissue and lymphocytes. DNA was extracted from paraffin-embedded RF tumor tissues and was used to amplify a portion of the SRV-2/WA genome encoding an open reading frame, 3'orf. Since 3'orf is encoded in an alternate reading frame within the outer membrane glycoprotein (gp70) env gene, sequences of 3'orf also provide information on a portion of env gp70. To define the changes that have occurred in this portion of the viral genome, we compared the nucleotide and deduced amino acid sequences amplified from the various tumor tissues. The paraffin section PCR revealed that all samples containing amplifiable DNA harbored SRV-2/WA genomes. DNA sequence analysis showed that the 3'orf/gp70 portion of the SRV-2/WA genome was nearly identical in all tissues but was distinguishable from the prototype SRV-2/OR isolate from Celebes black macaques (M. nigra). A T-cell activating domain (Env residues 233-249) was 100% conserved in the SRV-2/WA isolates.