The cost of 2 g ampicillin/1 g sulbactam given IV piggyback qid was compared with 900 mg clindamycin admixed with 1.5 mg/kg gentamicin given IV piggyback tid for the treatment of perforated or gangrenous appendicitis in 116 patients. Fifty-eight ampicillin/sulbactam-receiving patients incurred greater costs for IV supplies (+104.6/patient vs +67.9/patient) and nursing administration costs (+16.5/patient vs +10.7/patient). On the other hand, pharmacist and technician preparation costs were greater for the 58 clindamycin/gentamicin-receiving patients (+15.4/patient vs +13.3/patient). The clindamycin/gentamicin-receiving patients also incurred additional changes for laboratory fees and pharmacokinetic monitoring--+18.7/patient and +36.1/patient, respectively. When incorporating all cost parameters, there were no statistically significant differences in mean total drug therapy costs between the two treatment regimens--+433.3 +/- +58.5/patient for ampicillin/sulbactam and +373.8 +/- +86.2/patient for clindamycin/gentamicin.