Techniques for the isolation and characterization of SV40 DNA replicative intermediates have been devised. These intermediates are being employed as templates for in vitro DNA replication systems. The mechanism of formation of SV40 oligomeric DNA is presently under investigation. It appears that catenated dimers arise by replication errors during failure to segregate the two newly synthesized daughter molecules. Circular dimers may arise from recombination events between these catenated dimeric molecules. The proteins involved in the replication of viral DNA are being isolated and characterized.