The obvious costs of antibiotic treatment include drugs, equipment with which to give them, and assays. Less obvious, but more important, are the costs of quality control to ensure safe and effective treatment. The more complex the regimen, the more expensive the quality control. None the less, there is considerable variation in both assay price and number of assays/patient. Our data show that the drug costs of a regimen such as ampicillin plus gentamicin plus metronidazole are outweighed by the costs of quality assurance to prevent drug toxicity and charges of malpractice. Trials show that monotherapy with various beta-lactams is more cost effective than aminoglycoside combinations for surgical infections. Compounds such as piperacillin/tazobactam, a new beta-lactam/beta-lactamase inhibitor combination that is classed as monotherapy, have the potential to solve many of these economic problems. Several completed and continuing clinical studies are showing that monotherapy is as effective as combination treatment. Cost studies in the future are likely to confirm the economic advantages of monotherapy.