Mice infected intracerebrally (ic) or intraperitoneally (ip) with Pseudomonas aeruginosa were treated subcutaneously with graded doses of five antibiotics all of which displayed antipseudomonal activity in vitro. The analysis of MIC, ED50 in the ip infection model (ED50-ip) and that in the ic system (ED50-ic) for each agent showed: a) gentamicin, the most active agent in vitro, was also active in vivo, with an ED50-ic nearly six-fold higher than ED50-ip; b) ceftazidime, considerably active both in vitro and in vivo, showed comparable values of ED50 in the two systems; c) ceftriaxone, also active in vitro, showed limited activity in vivo, its two ED50 values being similar; d) aztreonam, moderately active both in vitro and in vivo, showed comparable activity in the two infection models; e) carbenicillin, the least active agent in vitro, gave poor results in vivo, with an ED50-ic 2.6-fold higher than ED50-ip.