The in vitro activity of the new semisynthetic ureidopenicillins azlocillin (AZ) and mezlocillin (MZ), and of the new cephalosporin cefsulodin (CEF) were determined against 50 carbenicillin-sensitive (CARS) and 50 carbenicillin-resistant (CARr) P. aeruginosa clinical isolates. In the CARS group the most active antibiotics are AZ and CEF with MICs between 0.5 and 8 μg/ml. Ticarcillin (TIC) and MZ showed more activity than CAR with MICs from 2 to 32 μ/ml. In this group there is a predominance of pyocine-type groups 1 and 3. In the CARr group, AZ is the most active antibiotic at low concentrations. At 64 μg/ml of CEF, 72% of strains are inhibited as compared to 70% with AZ, 62% with MZ and 50% with TIC. In this group there is predominance of non-typeable strains. Interaction with mecillinam (MEC) and these antibiotics was studied on three different culture media (MH, NIH and DST). There were few cases of synergism with MEC and TIC combinations, mostly on DST medium. No appreciable synergism was found with other combinations of antibioti