The aminoglycosides sisomicin, gentamicin, tobramycin, amikacin and kanamycin are highly active against staphylococci including the penicillinase-positive strains. Sisomicin is more effective than amikacin and kanamycin. Mixed infections with staphylococci and Enterobacteriaceae or Pseudomonas aeruginosa are thus on indication for treatment with sisomicin or other aminoglycosides. Infections with E. coli, Enterobacter, susceptible Klebsiella, and susceptible Pseudomonas strains can be treated with sisomicin, gentamicin or tobramycin. In such cases sisomicin is the most effective antibiotic because of its high antimicrobial activity. In infections with these organisms amikacin can also be used for treatment especially if there is resistance to other aminoglycosides. In hospital-acquired infections with Serratia marcescens amikacin and sisomicin are the drugs of choice. Both aminoglycosides have to be given in high doses in infections with Serratia because of the high inhibitory concentration for Serratia. Sisomicin demonstrates a high antimicrobial activity particularly against indole-positive Proteus species such as Proteus vulgaris and Proteus morganii, Enterobacter, and gentamicin-sensitive Pseudomonas strains. In infections with Pseudomonas aeruginosa tobramycin is the most effective bactericidal antibiotic. Amikacin is the drug of choice against gentamicin-resistant Pseudomonas strains which are also not infrequently resistant to other aminoglycosides. The low proportion of resistance to sisomicin of 7,6% in 370 organisms is only exceeded by amikacin with a rate of 0,6% (resistance to tobramycin 11,4%, gentamicin, 13,2% and kanamycin 42,4%). The low rate of resistance and the high antimicrobial activity are essential advantages of sisomicin.