Stability and costs of a mixture of clindamycin and gentamicin in a single carrier solution for every-eight-hour i.v. administration were analyzed. Duplicate solutions containing clindamycin 900 mg (as the phosphate salt) and gentamicin 100 mg (as the sulfate salt) in 5% dextrose injection 150 mL in flexible plastic containers were prepared. Samples withdrawn at 0-24 hours were visually inspected and assayed for content of both drugs. Clindamycin concentrations were determined by high-performance liquid chromatography and gentamicin concentrations by fluorescence-polarization immunoassay. Preparation and administration times were measured in two trials to determine cost differences between every-eight-hour administration of the two-antibiotic mixture and separate administration of the two drugs (four daily doses of clindamycin and three of gentamicin). No precipitate was seen. Concentrations of both drugs after 24 hours were greater than 90% of initial concentrations. Total daily costs (per patient) for labor, drug, and carrier solution were $3.08 less when the two drugs were mixed and administered every eight hours. At the concentrations studied, clindamycin and gentamicin mixed together in 5% dextrose injection are stable for 24 hours. Administration of this drug mixture every eight hours is safe and less costly than separate administration of the two drugs.