Abstract: A recombinant plasmid useful for the production of large-insert, stably maintained, episomes in mammalian cells is disclosed, along with cells, DNA libraries, and methods of using the same. The plasmid comprises a lymphotrophic herpes virus segment (e.g., an Epstein-Barr virus segment) containing an origin of plasmid replication (oriP) and a heterologous insert segment linked to the lymphotrophic herpes virus segment. The heterologous insert segment has a length of at least 100 kilobases. In a preferred embodiment, the lymphotrophic herpes virus segment is capable of producing infectious virions in a suitable host cell, and the lymphotrophic herpes virus segment has regions deleted so that the recombinant plasmid retains the capability of producing infectious virions in the host.