Bismuth-dithiol mixtures are proven antimicrobial agents with unknown mechanism(s) of action. We show that select bismuth-dithiol solutions inhibit the Escherichia coli rho transcription termination factor. Rho is an essential enzyme in most Gram-negative prokaryotes and without rho function the cells are not viable. Bismuth complexes with 2,3-dimercapto-1-propanol (BiBAL) (3:1 solutions) functioned as a noncompetitive inhibitor with respect to ATP in the rho poly(C)-dependent ATPase assay (I50=60 microM) and as a competitive inhibitor with respect to ribo(C)10 in the poly(dC)-ribo(C)10-dependent ATPase assay. The minimum inhibitory concentration (MIC) of bacterial growth for BiBAL (3:1) in the liquid culture assay using E. coli W3350 was 16 microM. Using the tnaA/lacZ fusion reporter assay we showed that sublethal amounts (3 microM) of BiBAL (3:1 solution) led to a small increase (37%) in in vivo beta-galactosidase activity in E. coli SVS1144, which corresponds to antitermination of the tna operon as a result of rho inhibition. We concluded that BiBAL was a potent in vitro rho inhibitor but its effect on in vivo rho processes was modest indicating that other mechanisms contributed to the antibacterial activity of BiBAL. Our study suggests that structural changes in the dithiol unit that provide greater bismuth binding may improve rho specificity, a macromolecular target not previously recognized for bismuth therapy.