2,570 strains of tuberculosis bacilli were tested against 12 tuberculostatics and also against the recently marketed compound Rifampicin. The culture medium employed was Löwenstein-Jensen and the solvent for Rifampicin was either DMSO or dimethylformamide; both were equally efficient. Due to a certain instability of the medicament in the culture medium, the minimal inhibitory concentrations determined varied between 0.5 and 16 µg Rifampicin/ml culture medium. The highest percentage was 4 µg/ml; in the case of the control strain H 37 Rv it was 4–8 µg/ml. 18 of the strains tested were resistant to Rifampicin. The possibility of the rapid development of resistance to Rifampicin, if therapy is carried out without due care, is indi