The external envelope glycoprotein (gp46) and transmembrane glycoprotein (gp21) of human T-cell lymphotropic virus type I (HTLV-I) were isolated from lysates of HTLV-I-infected HUT-102 cells by affinity chromatography. Fifty ml aliquots of packed HUT-102 cells were extracted with 1% Triton X-100, and lysates were treated sequentially with an affinity column containing IgG from an HTLV-I+ human subject followed by chromatography of the bound fraction over a lentil lectin column. The identity of the purified envelope proteins was confirmed with a human monoclonal antibody (0.5 alpha) to gp46 and with rabbit antisera raised to a synthetic peptide from the C-terminus of gp21. Affinity-purified envelope glycoproteins were bound to microtiter wells and used in radioimmunoassay to detect murine and human anti-envelope antibodies to gp46 and gp21 molecules.