The therapeutic failure of aminoglycosides in leukopenic mice was studied. Pseudomonas aeruginosa isolated from experimentally infected leukopenic mice was analyzed for susceptibility to gentamicin. Bacterial population analyses were performed before, during, and after therapy with high doses of gentamicin. Inocula from numerous strains, including P. aeruginosa strain ATCC 27853, harbored slowly growing aminoglycoside-resistant subpopulations. In leukopenic mice (in contrast to normal mice) receiving gentamicin, the breakthrough growth of these subpopulations occurred 6 hr after treatment. Resistance to gentamicin reversed in vivo when gentamicin treatment was stopped. The emergence of gentamicin-resistant subpopulations was successfully suppressed by a combined treatment regimen of gentamicin and ticarcillin. Gentamicin-ticarcillin synergy in these infections is due, in part, to antimicrobial suppression of slowly growing aminoglycoside-resistant subpopulations of bacteria.