Time-kill curves were used to assess the relative in vitro efficacy of the early interaction of three semisynthetic penicillins with two aminoglycosides against 48 Enterobacteriaceae strains. The most efficacious combinations were piperacillin plus amikacin, which demonstrated synergism (>2 logs of increased kill after 7 h of incubation) against 43 of 48 (901%) strains, and piperacillin plus gentamicin, which exhibited synergism against 25 of 48 (52%) strains. With the combinations of carbenicillin or ticarcdillin plus amikacin or gentamicin, early synergistic killing was demonstrated against only 12 to 29% of the strains.Semisynthetic penicillins such as ampicillin, carbenicillin, and ticarcillin have been used for several years in the therapy of severe gramnegative baciliary infections. Piperacillin is a new semisynthetic penicillin which has demonstrated greater in vitro activity than carbenicillin or ticarcillin against Pseudomonas aeruginosa and members of the Enterobacteriaceae (1,5,13,15,16,19,20 Antibiotic synergism. The interaction between each of the three semisynthetic penicillins and each of the two aminoglycosides was evaluated by time-kill curves in Mueller-Hinton broth as described previously (6). A starting inoculum of 105 organisms per ml was prepared by appropriate dilution of an overnight broth culture. The concentrations of antibiotics in the flasks were selected to be at or below the MIC for each drug and within a clinically achievable range for each drug::125 pg/ml for each of the penicillins, s4 pg/ml for gentamicin, and <8 .g/ml for amikacin. The culture flasks (final volume, 20 ml) were incubated without agitation at 35°C, and 0.5-mi samples were removed at 0, 4, and 7 h for determination of colony counts with 10-fold serial dilutions. Early synergy is herein defined as a decrease of 100-fold or more in the number of 902 on May 11, 2018 by guest http://aac.asm.org/ Downloaded from