Retroviral gene transfer is presently one of the most powerful techniques for introducing stably heritable genetic material into mammalian cells (reviewed in ref. 1). One serious drawback of this technique, however, has been the difficulty in readily producing high-titer recombinant retroviruses. For many applications, such as infecting rare target cells or the majority of cells in tissue culture, the recombinant virus titer must be at least 10(6) infectious units/mL. Although one can usually obtain high-titer mixtures of recombinant and replication-competent retroviruses in a relatively short time, many applications such as cell marking studies or studying genes in vivo demand freedom from replication-competent virus.