The emergence of bacteria with R-factor-mediated resistance transferable to many strains of Pseudomononas aeruginosa, Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, and some Proteus and Providencia species has been reported in hospitals in the Rhine-Main region of Germany. In a comparative study, 1,250 strains belonging to 12 clinically important bacterial species were tested for susceptibility to gentamicin, sisomicin, tobramycin, and amikacin by the tube dilution methods. Gentamicin, sisomicin, and tobramycin can still be employed effectively for the treatment of serious gram-negative infections. However, some findings related to Enterobacteriaceae and P. aeruginosa and demonstration of transfer by R-plasmids among P. aeruginosa, E. coli, K. pneumoniae, Enterobacter, and S. marcescens point to less favorable developments in the future. nearly all bacterial strains tests, including those resistant to other aminoglycosides, were susceptible in vitro to amikacin. This highly valuable antibiotic should be employed only when specifically indicated, since indiscriminate use would favor selective emergence of resistant bacteria in the future.