From the initial investigations on the antibacterial spectrum of rifomycin S.V it was found that, contrary to the high sensitivity of the gram-positive bacteria, most gram-negative rods were relatively resistant to this antibiotic. Nevertheles rifomycin concentrations which inhibited the majority of the biliary tract of man and experimental animals, as rifomycin is concentrated and eliminated through the bile. These facts suggested the need of defining the bacteriostatic and bactericidal levels of rifomycin S.V., against pathogenic gram-negative enteric organisms, because of the importance of the Salmonella carriers and the unsolved problems of their treatment - and since some divergence were found in the few available reports on their sensitivity to rifomycin. Seventy seven strains of Salmonella typhi, and 88 other strains which included representative straign of most groups among the Enterobacteriaceae, were tested. The serial dilution technique in a tryptose- phosphate-glucose- broth was adopted. Most the Escherichia coli, Shigella and Proteus mirabilis cultures were inhibited within the range of 33-65 μg/ml of rifamycin S.V. whereas the mean concentrations for inhibition of most strains of other groups of enteric bacteria including the remaining species of Proteus, Klebsiella and Providencia cultures varied between 65 and 128 μg/ml of rifomycin. For Salmonella and Enterobacter in most cases the minimal inhibitory concentration was 128 μg/ml. More striking differences among the tested bacterial groups were found as related to the bactericidal action of rifomycin S.V. For Shigella and E. coli strains the bacteriostatic and bactericidal concentrations of rifomycin S.V. closely paralleled each other. As to the four Proteus species and Providencia strains, in spite of some variability in most instances the bactericidal concentrations was four times greater than the minimal inhibitory one. Finally, and unfortunately for the above mentioned purposes, in relation to Salmonella (S. typhi included) when given a 6 times large concentraction than the already high minimal inhibitory concentrations observed, such a concentration did not kill the strains tested.