Combination antibiotic therapy has been used mainly to broaden the antibacterial spectrum and prevent the development of resistance. Antibiotic combinations proven to be synergistic in vitro are associated with a significantly better in vivo response, particularly in the compromised host in whom traditional treatment combines an antipseudomonal penicillin plus an aminoglycoside. Several investigators have examined combining new agents, such as the third-generation cephalosporins (cefotaxime, ceftriaxone, ceftizoxime, ceftazidime, cefoperazone, and moxalactam), aztreonam, or the ureidopenicillins, with amikacin. When compared with combinations of an older cephalosporin, carbenicillin or ticarcillin, plus gentamicin or tobramycin, these newer combinations produce higher rates of clinically meaningful synergy and rapid enhancement of in vitro bactericidal activity against the difficult-to-treat Enterobacteriaceae (i.e., Serratia, Citrobacter, Enterobacter, Providencia, and indole-positive Proteus species). This effect, without any evidence of antagonism, has been reported even for strains moderately or completely resistant to the former antibiotics. Unsatisfactory and unpredictable synergistic interactions against both resistant and susceptible strains of Pseudomonas aeruginosa--the most difficult nosocomial pathogen to treat--have been noted with combinations of tobramycin or gentamicin plus cefotaxime, moxalactam, or cefoperazone. Conversely, the use of amikacin plus various beta-lactams against multi-resistant strains is more frequently synergistic. Agents have been observed to exhibit such synergy in the following order of activity, from most to least synergistic: ceftazidime, ceftriaxone, moxalactam, aztreonam, cefotaxime, azlocillin, cefoperazone, cefsulodin, and carbenicillin. The combination of amikacin plus imipenem or ciprofloxacin against strains of P. aeruginosa resistant to the former and moderately resistant to the latter was recently reported to have a low probability of synergy; the combination of two of the newer beta-lactams had mostly an unpredictable or even antagonistic result. In vitro studies have also demonstrated that high concentrations of the antipseudomonal penicillins can inactivate the aminoglycosides. Among the latter compounds, the inactivation order, from most to least inactivated, was as follows: tobramycin, gentamicin, netilmicin, and amikacin. To date, the reports of aminoglycoside inactivation by the newer cephalosporins have been rather contradictory; only moxalactam has been shown produce a significant decrease in activity.