In vitro activity of sisomicin and gentamicin was compared in serial dilution tests for 619 bacterial strains (Staphylococcus aureus, E. coli, Proteus mirabilis, Proteus vulgaris, Enterobacter, Klebsiella, Pseudomonas aeruginosa, Salmonella, Serratia marcescens). Mean MIC of sisomicin was lower by one geometrical dilution step compared with gentamicin for Pseudomonas, Proteus vulgaris, Klebsiella, and Serratia, while it was almost identical with the other species. Resistance (MIC greater than 5 microgram/ml) against sisomicin was observed in 2% of Pseudomonas strains, resistance against gentamicin in 8%. Ten healthy adult volunteers had serum peak levels (after i,m. injection of 40 mg and 80 mg sisomicin) of 2.7 and 3.2 microgram/ml. Urine recovery (in 24 hrs) was 76%. Continuous i.v. infusion of sisomicin or gentamicin (6.6 mg/hour in 7 healthy adult volunteers) yielded in serum levels of 0.64 and 1.03 microgram/ml respectively. Biological half-life (90 minutes), urine recovery (60% in the fourth hour), renal clearance and skin blister levels at the end of infusion were almost identical for both antibiotics; total clearance was somewhat higher with sisomicin than with gentamicin.