To detect surface-expressed protein, cells transfected with the appropriate envelope expression vector were detached from culture flasks using PBS +2 mM EDTA, and washed twice with PBS. 5.0610 5 cells were incubated with agitation for 1 hr at room temperature in 1 ml DMEM +10% FBS containing 15 mgml 21 immunoglobulin purified from the BLV anti-Env antibodyexpressing hybridoma supernatant, or 150 ml supernatant from the HTLV anti-Env antibody expressing hybridoma. Cells were washed twice with PBS and incubated in the dark at room temperature for 45 mins in DMEM containing anti-mouse FITC (Sigma) at 1/1,000 dilution. Cells were washed once with PBS and once with PBS +0.1% sodium azide before fixing with PBS +0.5% paraformaldehyde. Bound fluorescence was detected using a FACScan flow cytometer (Becton Dickinson).