A comparative in vitro investigation of the new aminoglycoside netilmicin and 4 other aminoglycosides (gentamicin, tobramycin, amikacin and kanamycin) was performed. Gentamicin and tobramycin resistant E. coli, Klebsiella spp. and Proteus mirabilis proved to be sensitive to netilmicin and amikacin. Of gentamicin and tobramycin resistant indole positive Proteus strains only a few were sensitive to netilmicin and amikacin. Amikacin was the most active drug against Providencia spp.; Pseudomonds aeruginosa resistance to gentamicin usually paralleled resistance to netilmicin but only rarely to amikacin. Netilmicin was not active against kanamycin resistant Acinetobacter calco-aceticus strains. When the bactericidal activity of the 5 antibiotics was compared no differences in speed of killing were found.