Recently, a 0.8 kb region of the broad-host-range plasmid RK2 has been shown to be sufficient to stabilize plasmids in a vector-independent, broad-host-range manner under some but not all growth conditions (Roberts, R. C. & Helinski, D. R. (1992). J. Bacteriol. 174, 8119-8132). This region encompasses the parDE operon, which encodes the small proteins ParD and ParE, both of which are required for the plasmid stabilization. This paper demonstrates that the 0.8 kb region encodes the capacity to inhibit cell growth of Escherichia coli, presumably of those bacteria that have lost plasmids carrying this stabilization region, and this inhibition appears to be associated with cell killing and bacterial cell filamentation. A good correlation was observed between the capacity of wild-type and mutated 0.8 kb regions to promote stable maintenance of a temperature-sensitive RK2 replicon plasmid and to inhibit bacterial cell division under specified medium conditions. The properties of the wild-type and mutant 0.8 kb regions further indicate that the ParE protein is responsible for the growth inhibition and the ParD protein neutralizes the toxic activity of the ParE protein. This is consistent with the finding that the presence of the parD gene in trans destabilizes a temperature-sensitive RK2 replicon carrying a copy of the functional 0.8 kb region. This destabilization appears to be the result of ParD protein-mediated suppression of growth inhibition, thus allowing survival of cells that have lost the temperature-sensitive plasmid. These observations indicate that the 0.8 kb sequence of RK2 encodes a growth inhibition function that is likely to play a role in the plasmid stabilization.