In a comparison of the antibacterial in-vitro activity of sisomicin, gentamicin, and tobramycine, sisomicin showed a higher activity against E. coli, indole-positive Proteus spp. and organisms of the Klebsiella-Enterobacter-Serratia group, whereas tobramycine was superior against Pseudomonas spp. However, the differences in activity between sisomicin and gentamicin were only within one step of dilution which is hardly sufficient to guarantee a basic superiority of one or the other preparation for clinical purposes. In 12 healthy probands serum levels and renal elimination of sisomicin and gentamicin after a single intramuscular injection of 1 mg/kg body weight were investigated in a randomised change-over trial. With corresponding serum levels, almost identical elimination half-lives of 109 and 111 minutes, and recovery values of 84 and 88.4%, respectively, there were no significant differences in the pharmacokinetics of sisomicin and gentamicin. Thus in the antibacterial treatment of gentamicin-sensitive gram-negative organisms sisomicin presents an equally good alternative. With organisms only moderately sensitive or resistant to gentamicin, sisomicin, tobramycine and amikacin should be additionally tested.