A simian immunodeficiency virus (SIV) designated SIVMne was isolated from a pig-tailed macaque with lymphoma housed at the University of Washington Regional Primate Research Center, Seattle. To better establish the relationship of SIVMne to other immunodeficiency viruses, we purified and determined the partial amino acid sequences of six structural proteins (p1, p2, p6, p8, p16, and p28) from SIVMne and compared these amino acid sequences to the translated nucleotide sequences of SIVMac and human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2). A total of 125 residues of SIVMne amino acid sequence were compared to the predicted amino acid sequences of the gag precursors of SIV and HIVs. In the compared regions 92% of the SIVMne amino acids were identical to predicted residues of SIVMac, 83% were identical to predicted residues of HIV-2, and 41% were identical to predicted residues of HIV-1. These data reveal that the six SIVMne proteins are proteolytic cleavage products of the gag precursor (Pr60gag) and that their order in the structure of Pr60gag is p16-p28-p2-p8-p1-p6. Rabbit antisera prepared against purified p28 and p16 were shown to cross-react with proteins of 60, 54, and 47 kilodaltons present in the viral preparation and believed to be SIVMne Pr60gag and intermediate cleavage products, respectively. SIVMne p16 was shown to contain covalently bound myristic acid, and p8 was identified as a nucleic acid-binding protein. The high degree of amino acid sequence homology between SIVs and HIV-2 around proven proteolytic cleavage sites in SIV Pr60gag suggests that proteolytic processing of the HIV-2 gag precursor is probably very similar to processing of the SIV gag precursor. Peptide bonds cleaved during proteolytic processing of the SIV gag precursor were similar to bonds cleaved during processing of HIV-1 gag precursors, suggesting that the SIV and HIV viral proteases have similar cleavage site specificities.