Abstract: High titre helper-free recombinant retrovirus are produced by (a) growing a transfected host cell, produced by transfecting a eukaryotic host cell with a recombinant vector capable of stable episomal maintenance in the host cell, in a medium under conditions whereby the recombinant vector is stably maintained as an episome in the transfected host cell and transcripts of said vector form, with retroviral gag, pol and env gene products, an infectious retrovirus; and (b) isolating from the medium helper-free infectious retrovirus formed in the transfected host cell. The recombinant vectors comprise (i) a retroviral construct comprising an exogenous gene; (ii) a eukaryotic origin of replication sequence providing a substrate for replicase activity capable of replicating the vector in the host cell; and, (iii) a copy control sequence providing a substrate for a copy control activity capable of maintaining the vector at a stable copy number in the host cell.