The occurrence of a postantibiotic effect (PAE) and postantibiotic leukocyte enhancement (PALE) of tobramycin, a natural aminoglycoside clinically used since the early 1970s, has been investigated in comparison to gentamicin on recent clinical isolates of Staphylococcus aureus (both methicillin-susceptible and -resistant strains) and of Gram-negative fermenting and non-fermenting rods. A concentration-dependent PAE was observed with both antibiotics, regardless of the bacterial species used, with some variability based on their intrinsic resistance. Tobramycin, at concentrations equal to or higher than the minimum inhibitory concentration (MIC), exhibited a rather long PAE (ranging from 1.9 to 10.9 h) which was often significantly longer than that observed with gentamicin (ranging from 1.0 to 7.5h). Moreover, pre-exposure to tobramycin led to enhanced polymorphonuclear leukocyte phagocytosis and killing with a 2- to 27-fold increase in activity compared to controls. These results suggest that tobramycin might be conveniently used with once-daily dosing for the treatment of infections due to sensitive pathogens.