The antibacterial activity of gentamicin and sisomicin was studied in 148 recent clinical isolates of Proteus obtained from patients hospitalized in Athens. Both gentamicin and sisomicin were found to be active with sisomicin generally being the more active of the two; P. mirabilis strains were less susceptible than the indole-positive strains, but P. mirabilis organisms isolated from the respiratory tract were more sensitive to sisomicin than those isolated from the urine. Susceptibility testing with the two aminoglycoside antibiotics was affected by inoculum size and by the test broth used. Sisomicin sensitivity testing with the disc-agar diffusion method and broth dilution method was reliable for the indole-positive strains of Proteus but did not separate all sensitive from resistant strains of P. mirabilis. An in vitro synergism was demonstrated between sisomicin and the semisynthetic penicillin, ticarcillin.