Ml3 phages do not lyse their host, but are released from infected cells as the cells continue to grow and divide. Cells infected with Ml3 have a longer replication cycle than uninfected cells, and as the infection proceeds, areas of slower-growing cells can be visualized as turbid plaques on lawns of E. coli (1). Well-separated plaques contain cells infected with phages derived from a single transformation event and these can be picked and regrown to provide pure stocks of recombinant phage particles and DNA.