The present study deals with the mechanism by which plasmid pCED3 interferes with the growth of Bacillus subtilis. Plasmid pCED3 was constructed from pUB110 and pBR322 and contains the lacZ gene attached to the B. subtilis tms promoter. Plasmid derivatives that contain mutations in the tms promoter were used to examine the effect of promoter strength on cell growth, plasmid stability, the amount of plasmid DNA per cell and the activities of plasmid-encoded enzymes, i.e., beta-galactosidase and kanamycin nucleotidyltransferase (KNT). Efficient lacZ transcription directed from the tms promoter resulted in reduction in growth rate and plasmid stability without an increase in beta-galactosidase activity. The amount of plasmid DNA varied between 6.6 and 12.9 pmol per mg cell protein and showed no clear correlation with the strength of the tms promoter. Transcription from the tms promoter inhibited the expression of the plasmid-encoded kanamycin resistance gene resulting in the reduction of both beta-galactosidase activity and growth rate in the presence of kanamycin. These results suggest that the negative effect on B. subtilis growth exerted by pCED3 results at least partly from a decrease in kanamycin resistance by plasmid-bearing cells.