Enhancement of viral infectivity in a defined incubation time period conferred by the presence on HIV-1 of host-derived MHC-II glycoproteins. HeLa-CD4-LTR-β-gal cells were incubated for 15 minutes at 37°C with 100 TCID₅₀ of progeny virus produced by HIV-1_IIIB–infected RAJI-CD4 (▪) or RM3-CD4 (□) cells. Cells were washed several times to remove unadsorbed viruses and incubated for 48 hours in a 37°C, 5% CO₂ incubator. The number of blue-stained cells were counted using a light microscope. Results shown are the mean ± SD of triplicate determinations. The number of blue cells in mock-infected cells was always less than 5.