Turbidimetric and morphologic responses to eight antipseudomonal beta-lactam antibiotics were compared for selected strains of Pseudomonas aeruginosa with different susceptibilities to carbenicillin. In conventional minimal inhibitory concentration tests, all of the newer antibiotics appeared more active than carbenicillin, and apalcillin and cefsulodin had the greatest overall activity. However, in turbidimetric tests the activity of apalcillin and three other N-acyl penicillins (azlocillin, mezlocillin, and piperacillin) was inferior to that of carbenicillin and the other agents. The N-acyl penicillins were also all susceptible to intrinsic pseudomonal beta-lactamase, so that dense bacterial populations inactivated these antibiotics in concentrations of greater 128 micrograms/ml during overnight incubation. Against carbenicillin-resistant strains, carbenicillin, ticarcillin, and sulbenicillin were the least active antibiotics, and cefsulodin had the best overall activity. Turbidimetric monitoring highlights the problems of interpreting the results of conventional minimal inhibitory concentration tests, particularly when large inocula are involved.