We examined fusion mediated by the human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2) envelope glycoproteins under various experimental conditions. Incubation of HeLa cells expressing HIV-2(ROD) and HIV-2(SBL/ISY) envelope glycoproteins with HeLa-CD4 target cells resulted in fusion at temperatures >/=25 degrees C whereas fusion with cells expressing HIV-1(Lai) occurred only at >/=31 degrees C. HIV-2 envelope glycoprotein-mediated fusion proceeded in the absence of Ca(2+) in the culture medium, whereas HIV-1 fusion required Ca(2+) ions for fusion. In contrast to HIV-2 envelope glycoprotein fusion, incubations in the presence of the 0.5 microM cytochalasin B completely inhibited HIV-1 envelope glycoprotein-mediated fusion. Our results suggest that in contrast to HIV-2, HIV-1 fusion is dependent on dynamic processes in the target membrane.