Stable maintenance of single-copy plasmids, such as P1 and F, requires gene products produced by the plasmid, a cis-interacting site on the plasmid, and gene products of the bacterial cell. The plasmid encoded components have been well identified and we now seek to study the bacterial encoded components. To this end we have set up a screen that allows us to identify cells that can no longer stably maintain the F plasmid. Sixty independent mutant strains of E.coli have been isolated. We are currently examining each of these mutants for their ability to maintain the P1 plasmid and for the kinetics of plasmid loss in each of these mutants.