Abstract: This invention is directed to construction of recombinant plasmids that contain the Epstein-Barr virus ("EBV") genetic information involved in the immortalization of human lymphocytes, but that lack one or more genes implicated in EBV lytic replication. The recombinant plasmids have the advantage of being able to immortalize human lymphocytes, while being incapable of generating infectious EBV particles. The invention also provides human lymphocytes that have been successfully infected and immortalized by the recombinant plasmids. These human lymphocyte clones can be used as cellular factories to produce desired proteins and as delivery vehicles for gene therapy. Additionally, the invention establishes a method by which desired proteins can be produced in immortalized cellular factories, without generating infectious EBV particles. These proteins can be the normal cellular products of immortalized human lymphocytes, or they can be proteins encoded by foreign genes, which have been cloned into the lymphocytes.