Measurement of TF-1 cell proliferation after incubation with HTLV-II virus. TF-1 cells were deprived of IL-3 for 24 hours and then seeded at a concentration of 2.5 × 10⁵ cells/well in a volume of 300 μL of culture medium. At this time, cells were either supplemented with IL-3 (subsequently added every 4 days) or treated with the various stimuli listed in the bottom panel. Treatments with HTLV-II strains were for 24 hours. Cell proliferation was measured by³H-thymidine incorporation (cpm in the ordinate) over time (days in the abscissa).