In admixtures containing clindamycin and either aztreonam, ceftazidime, ceftriaxone, or piperacillin in either 5% dextrose injection (D5W) or 0.9% sodium chloride injection (NS), the stability of each drug was studied. Each of the following combinations of drugs was added to 100-mL glass bottles of base solution: clindamycin phosphate 0.9 g and aztreonam 2.0 g, clindamycin phosphate 0.9 g and ceftazidime sodium 2.0 g, clindamycin phosphate 1.2 g and ceftriaxone sodium 2.0 g, and clindamycin phosphate 0.9 g and piperacillin sodium 4.0 g. Duplicate samples were prepared. Admixtures containing each single drug were also tested. Samples were visually inspected and tested for pH and drug concentration immediately after mixing and at 1, 4, 8, 12, 24, and 48 hours of storage in room temperature and light. Drug concentrations were determined by high-performance liquid chromatographic assay methods. Ceftriaxone retained greater than 90% of its original concentration for 24 hours in single-drug admixtures in NS, for eight hours with clindamycin in NS, and for one hour with clindamycin in D5W. Ceftazidime retained greater than 90% potency for 24 hours with clindamycin in D5W. In all other test admixtures, all drugs were stable for 48 hours. Under the conditions studied, clindamycin is compatible in the admixtures tested with aztreonam and piperacillin. Admixtures of clindamycin and ceftazidime in D5W should be used within 24 hours at room temperature. Clindamycin and ceftriaxone can be mixed in NS if administered within eight hours, but ceftriaxone is stable for only one hour in combination with clindamycin in D5W.