The stability and compatibility of clindamycin phosphate and ceftizoxime sodium, cefoxitin sodium, cefamandole nafate, or cefazolin sodium in two intravenous solutions were studied. Each antibiotic alone as well as each of the four two-drug combinations were examined when mixed in duplicate 100-mL glass bottles of 5% dextrose and 0.9% sodium chloride injections. Antibiotic concentration, pH, and visual appearance were recorded at the time of preparation and at 1, 4, 8, 12, 24, and 48 hours. Antibiotic concentrations were assessed with drug-specific high-performance liquid chromatographic assays. Decreases in concentration of 10% or more from the original concentration were considered to indicate instability. All the single antibiotic solutions were stable for 48 hours. Clindamycin was stable in all combinations except with ceftizoxime in 0.9% sodium chloride injection, which measured 89.3% of its original clindamycin concentration at 48 hours. All the cephalosporins mixed with clindamycin were stable for 48 hours. Clindamycin is stable for at least 48 hours when mixed with cefoxitin sodium, cefamandole nafate, or cefazolin sodium in either 5% dextrose or 0.9% sodium chloride injections and for at least 24 hours when mixed with ceftizoxime sodium in 0.9% sodium chloride injection.