The invention provides a novel retroviral packaging system, in which retroviral packaging plasmids and packagable vector transcripts are produced from high expression plasmids after stable or transient transfection in mammalian cells. High titers of recombinant retrovirus are produced in these transfected mammalian cells and can then transduce a mammalian target cell by cocultivation or supernatant infection. The methods of the invention include the use of the novel retroviral packaging plasmids and vectors to transduce primary human cells, including T cells and human hematopoietic stem cells, with foreign genes by cocultivation or supernatant infection at high efficiencies. The invention is useful for the rapid production of high titer viral supernatants, and to transduce with high efficiency cells that are refractory to transduction by conventional means.