The parental DNA of bacteriophages λ and φX174 becomes associated with a host cell component after infection. This host cell component has the properties of a lipoprotein and is probably the cell membrane. There is a minimum of 60 to 80 sites on the membrane for attachment of parental λ and φX DNA, but the sites are not homologous for the two phages. Replication of parental λ and φX DNA occurs on the membrane, but, at high multiplicity, parental DNA of each is released into the cytoplasm. The semiconservative replication of φX parental replicative form DNA is limited to one or a few genomes per cell, whereas more than 60 parental λ genomes per cell can be replicated during the infective process. © 1969.