The activity of ticarcillin, BL-P1654, and carbenicillin was compared in vitro using a microtiter tube dilution test in Mueller-Hinton broth against 50 recent clinical isolates each of Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, Klebsiella species, Enterobacter species, Proteus species, and Pseudomonas aeruginosa. Bactericidal end points were determined using a modified Steers replicator. Ticarcillin was generally two to four times more active against all organisms tested except S. epidermidis against which BL-P1654 was most active. Median minimum inhibitory concentrations in micrograms per milliliter were for S. aureus: ticarcillin (6.2), carbenicillin (12.5), BL-P1654 (25); for S. epidermidis: BL-P1654 (1.6), ticarcillin (3.2), carbenicillin (3.2); for E. coli: ticarcillin (3.2), BL-P1654 (6.2), carbenicillin (6.2); for Klebsiella sp.: >100 for all three drugs; for Enterobacter sp.: ticarcillin (3.2), carbenicillin (6.2), BL-P1654 (12.5); for Proteus sp.: ticarcillin (1.6), carbenicillin (1.6), BL-P1654 (3.2); for P. aeruginosa: ticarcillin (31), BL-P1654 (62), carbenicillin (125). Bactericidal end points were dependent on both the drug and the species but were in general no more than twofold more than the minimum inhibitory concentration with the exception of BL-P1654 against P. aeruginosa. BL-P1654 was bactericidal for only 60% of the strains tested at a concentration of 500 gg/ml.Carbenicillin has been found, in the years since its introduction, to be a remarkably useful drug in certain patients with infections caused by Pseudomonas aeruginosa, Serratia marcescens, and other selected susceptible pathogens. However, the emergence of carbenicillin-resistant strains and the risk of electrolyte and coagulation problems associated with high-dose therapy of sytemic infections have limited its clinical use.BL-P1654 is the sodium salt of 3-guanylureido-phenyl-acetamino-penicillanic acid, with a spectrum similar to that of carbenicillin. BRL-2288, ticarcillin, is the disodium salt of a-carboxy-3-thienylmethyl penicillin, also with a spectrum similar to that of carbenicillin. We compared the in vitro activity of these three drugs against a number of recent clinical isolates.MATERIALS AND METHODS Fifty isolates each of Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, Klebsiella species, Enterobacter species, Proteus species, Pseudomonas aeruginosa, and 10 isolates of enterococci were studied. The organisms were isolated in the Clinical Microbiology Laboratory of Colorado General Hospital, University of Colorado Medical Center, having been recovered from specimens of blood, urine, sputum, and other infected sites submitted from patients and were identified by standard methods under the direction of L. Barth Reller. Minimum inhibitory concentrations were carried out using microtiter tube dilution tests in Mueller-Hinton broth, adding 0.05 ml of a 10-4 dilution of an overnight Mueller-Hinton broth culture of each organism to be tested to 0.05 ml of diluted antibiotic in a micr...