Proliferation of IL-2–dependent cells induced by IL-2–displaying retroviruses. F7 cells were arrested in Go/G1 by overnight deprivation of IL-2. Cell proliferation was then quantified 3 or 7 days later after culture in media conditioned with either 10 U of recombinant IL-2 (rIL-2), 13 μL of viral supernatants containing nonenveloped retroviruses (−), 13 μL of viral supernatants containing retroviruses coated with amphotropic envelopes (A), or 13 μL of viral supernatants containing retroviruses coated with IL-2 chimeric envelopes (IL2-SU). The values shown are the means ± SD of four separate experiments.