Lentiviral vectors have been advocated to be effective vehicles for the delivery and stable expression of genes in nondividing primary cells. However, certain cell types, such as resting T lymphocytes, are resistant to infection with HIV-1. Establishing parameters for stable gene delivery into primary human lymphocytes and approaches to overcome the resistance of resting T cells to HIV infection may permit potential gene therapy applications, genetic studies of primary cells in vitro, and a better understanding of the stages of the lentiviral life cycle. Here we demonstrate that an HIV-1-derived vector can be used for stable delivery of genes into activated human T cells as well as natural killer and dendritic cells. Remarkably, a sizeable fraction of resting T cells was stably transduced with the HIV-1 vector when cultured with the cytokine interleukin (IL)-2, IL-4, IL-7, or IL-15, or, at a lower level, with IL-6, in the absence of any other stimuli. Resting T cells stimulated with these cytokines could also be infected with replication-competent HIV-1. To test the utility of this system for performing structure-function analysis in primary T cells, we introduced wild-type as well as a mutant form of murine CD28 into human T cells and showed a requirement for the CD28 cytoplasmic domain in costimulatory signaling. The ability to stably express genes of interest in primary T cells will be a valuable tool for genetic and structure-function studies that previously have been limited to transformed cell lines. In addition, the finding that cytokine signals are sufficient to permit transduction of resting T cells with HIV may be relevant for understanding mechanism of HIV-1 transmission and pathogenesis.