Abstract
The maintenance of the plasmid pBR 322 in E. coli K12 C600 recB recC was investigated during successive discontinuous cultures. The percentage of tetracyclin resistant cells, the plasmid content and the β‐lactamase activity decrease during growth, indicating the segregation of plasmid‐free cells. The amount of plasmid‐free segregants which overgrow the plasmid‐containing bacteria, depends on the generation number of growth. By contrast, in C600 rec+ (pBR 322) the plasmid is maintained more stable during prolonged incubation. These results suggest the involvement of the recB recC‐gene product in plasmid maintenance.