The in vitro sensitivity of the H 37 RV strain of Mycobacterium tuberculosis to rifaximin and rifampicin was determined by evaluating both the antimycobacterial activity of the two rifamycin derivatives after oral administration to infected guinea pigs and the influence of rifaximin on the susceptibility to rifampicin of the isolated strain from spleen, lungs and liver. The results showed that after oral treatment with rifaximin no changes regarding the sensitivity of the strain to rifaximin and rifampicin were seen.