R. vannielii swarmer cells were more resistant to the effects of rifampicin than chain or budding cells. RNA polymerase purified from the different cell types showed no difference in susceptibility to rifampicin or in the major protein subunits as visualized by two-dimensional gel electrophoresis. The susceptibility of swarmer and budding cells to sodium deoxycholate gave a similar pattern to that found for rifampicin, with swarmer cells being more resistant than budding cells. This suggests that the changes in susceptibility to the inhibitors as swarmer cells differentiate into budding cells are due to changes in the permeability properties of the cell envelope that relate to changes in the role of cells during the cell cycle.