The therapeutic activities of orally administered FK482 were compared with those of reference antibiotics against systemic and local infections with a variety of bacteria in mice and rabbits. In systemic infections in mice, oral FK482 was almost as effective as oral cefaclor (CCL) and more effective than oral cephalexin (CEX) against Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis infections. However, FK482 afforded superior protective activity when given subcutaneously against E. coli infection in mice, and this activity was more potent than that of subcutaneously given CCL. In comparison with CCL, the reason that the in vivo activity of orally given FK482 against mouse systemic infections was weaker than had been anticipated from its potent in vitro activity was due to its poor oral absorption in mice. In local infections in rabbits, a species in which FK482 was better absorbed than in mice, FK482 was more effective than CCL, CEX or amoxicillin (AMPC). Against pneumonia induced by S. aureus or Streptococcus pyogenes, FK482 was as effective as AMPC and more effective than CCL in reducing the number of viable bacteria in the lungs of infected rabbits. In the oral treatment of experimental ascending pyelonephritis in rabbits, FK482 was superior to CCL and AMPC against methicillin-resistant S. aureus infection, as effective as AMPC and more effective than CCL against Enterococcus faecalis infection, and as effective as cefixime (CFIX) and more effective than CCL and AMPC against E. coli infection in reducing the number of viable bacteria in the kidneys and urine.