Kinetics of HIV-1 and HIV-2 Env-mediated fusion. HIV-1IIIB (squares), HIV-2SBL (triangles), and HIV-2ROD (circles) Env proteins were expressed in HeLa cells using vaccinia recombinants as described in Materials and Methods. Target SupT1 cells, labeled with calcein, were added to the plated HeLa cells, labeled with CMTMR, at various times during a two hour period at 37°C. The cells were then examined by fluorescence microscopy for dye transfer indicating cell-cell fusion. Lines represent fits to the sigmoidal equation f = a/(1-exp [-b(t - t1/2)]) using Sigmaplot (SPSS, Chicago). Values of time for half maximal fusion (t1/2) are 63 ± 6, 28 ± 2 and 23 ± 4 minutes for HIV-1IIIB, HIV-2SBL and HIV-2ROD, respectively.