The effect of microwave radiation on the stability of frozen cefoxitin sodium solutions was investigated. The i.v. fluids used as vehicles for the 1-g admixtures of cefoxitin sodium were 5% dextrose injection and 0.9% sodium chloride injection in 50-ml and 100-ml polyvinyl chloride flexible containers. The antibiotic small-volume parenteral solutions were frozen at -20 degrees C for 72 hours and thawed by microwave radiation. Before and after the freeze-thaw process, the solutions were observed for changes in appearance with a light and dark field visual surveillance technique. In addition, pH determinations were made with a microprocessor ionanalyzer, and high-performance liquid chromatographic determinations of concentration were performed. No significant drug concentration changes were detected and no visible changes were observed. The pH changes were minimal. Microwave radiation can reduce thawing time of antibiotic admixtures. In this study, the stability of cefoxitin sodium solutions was not affected by the freeze-thaw process.