Two-phase extraction in a system composed of dextran and polyethylene glycol was used to purify simian immunodeficiency virus, SIVMAC251 (32H isolate) from 25 l of culture supernatant. The virus partitioned to the interphase with 80% recovery of gag peptide p27 and reverse transcriptase and an about 25% recovery of the external env glycoprotein, gp148. The virus was treated with octylglycoside and its subcomponents separated. Two gag-p27 containing,fractions were obtained; gag-1, which also contained reverse transcriptase and nucleopeptides, and gag-2, which contained the major portion of the p27. The env gp148 was purified by chromatography through a series of lectin columns. The prepared materials are characterized by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and immuno- and lectin blotting.