Because persistence of infections associated with prosthetic material despite the use of appropriate antibiotics is a major clinical problem, the antimicrobial susceptibility of bacteria responsible for a chronic subcutaneous tissue cage infection in rat was investigated ex vivo. Three to 6 weeks after the initiation of infection, suspensions of two strains of Staphylococcus aureus recovered from the foreign body surface and surrounding fluid were exposed to either oxacillin, vancomycin, fleroxacin, gentamicin, or rifampin. The MBCs of these bacteria were markedly elevated, in most cases 128 to greater than 256 times higher than the MBC of batch culture S. aureus in either logarithmic or stationary phase. Kinetic studies showed the bacteria did not grow when incubated for 2 h in Mueller-Hinton broth, possibly reflecting dormancy. Their killing was slow and incomplete by all antibiotics at greater than 8 times their MIC. These data provide direct evidence of a decreased susceptibility of S. aureus to the killing effect of antimicrobials during chronic foreign body infections in vivo.