The purpose of this study was to evaluate the incompatibility of ceftriaxone with calcium-containing products, which had been the subject of an ALERT issued by the FDA. The influence of calcium ion concentration, storage temperature and shaking on the appearance and quantity of insoluble microparticles in mixtures of the two was examined using a light obscuration particle counter and a stereomicroscope. Appropriate volumes of 2% (w/v) calcium chloride solution were added to ceftriaxone sodium for injection (10 mg/ml) to make solutions with final calcium ion concentrations of 0.5-2.5 mmol/l, and stored at 20 degrees C, 25 degrees C, or 30 degrees C. The number of insoluble microparticles increased on storage when the calcium ion concentration of the sample was > or =2 mmol/l; it exceeded the permissible range at all temperatures by 1 h. The microparticles had a greater diameter at higher temperatures, although fewer microparticles were observed. The weight of precipitate increased as a function of both the calcium ion concentration and the temperature. The number of microparticles was also significantly increased by shaking. The number of microparticles in mixtures containing 1000 microg/ml ceftriaxone was significantly increased, even though concentrations of calcium ion was 1.25 mmol/l. Overall, not only calcium ion concentration, but storage temperature and shaking affected the extent of precipitation of ceftriaxone with calcium.