A monoclonal antibody (mAb),gp 21-34 which specifically recognizes Human T-cell lymphotropic virus type-II (HTLV-II) transmembrane envelope glycoprotein (p 20 EII) was coupled to Sepharose gel. A column packed with this gel specifically adsorbed p 20 E II and then desorbed with 3 M potassium thiocyanate. However, different virus type proteins HTLV- I p 20 E I are not adsorbed to the column. The mAb gp 21-34 coupled Sepharose column is useful for selective purification of p 20 E II and selective concentration of HTLV-II virions.