Effects of inoculum size on the comparative activity of carbenicillin, ticarcillin, mezlocillin, piperacillin and azlocillin against 20 strains of Pseudomonas aeruginosa known to be relatively resistant to carbenicillin were studied by using 10(4), 10(5) and 10(6) cfu/ml. Inoculum size markedly affected the susceptibility of Ps. aeruginosa to these antibiotics. An increase in the inoculum size from 10(4) to 10(5) cfu/ml had more effects on the MIC of the ureido-penicillins than the carboxy-penicillins. Mezlocillin, piperacillin and azlocillin with gentamicin or tobramycin showed mainly indifference. Synergism or antagonism were demonstrated against a few strains.