Serum sensitivity of HIV-1 vectors encoding eGFP and pseudotyped with the HIV-1 envelope HXB2 produced from cells cotransfected with ABO expression vectors. Vectors were harvested in serum-free medium and titer was determined. An MOI of 0.3 was incubated with 10% or 1% active sera from blood-typed donors for 1 hour at 37°C and plated onto NP2/CD4/X4 target cells. The number of GFP-expressing cells was determined by FACS 48 hours later and is shown as the percentage of infected cells compared with a matched heat-inactivated control. ▪ indicates type O serum; □, type A serum, and ▦, type B serum. Error bars indicate ± SEM.