The stability of aztreonam and clindamycin phosphate in intravenous admixtures was studied. Each of the following combinations of drugs was added to both 5% dextrose injection and 0.9% sodium chloride injection: aztreonam 20 mg/mL and clindamycin phosphate 6 mg/mL; aztreonam 20 mg/mL and clindamycin phosphate 3 mg/mL; aztreonam 10 mg/mL and clindamycin phosphate 6 mg/mL; and aztreonam 10 mg/mL and clindamycin phosphate 3 mg/mL. One of each of these admixtures was stored at 22-23 degrees C for 48 hours and at 4 degrees C for seven days. At various storage times the admixtures were inspected for visual changes and 1-mL samples were examined microscopically for crystalline and particulate matter, tested for pH, and assayed using high-performance liquid chromatography. No visual changes were observed. The pH of admixtures decreased only slightly during storage. Concentrations of aztreonam and clindamycin phosphate under both storage conditions decreased by less than 10%. Intravenous admixtures of aztreonam and clindamycin phosphate at the concentrations studied are stable for at least 48 hours at 22-23 degrees C and at least seven days at 4 degrees C.