Abstract
Poyoma DNA replication is initiated predominatly at a site which is 29% from the EcoRI cleavage site. Molecules replicating from this site, after digestion with EcoRI appear as linear structures with a double stranded loop centered at the origin of replication. These forms constitute 90% of all replicating intermediates. Approx. 10% of the replicating intermediates of polyoma and SVP40 DNAs occur as Y-forms after treatment with EcoRI. These structures have probably resulted from unidirectional replication initiated at an additional origin of DNA replication which is located near the EcoRI cleavage site on the genomes of these viruses.