Cephalothin sodium (CET-Na) in crystals can be obtained by freezing the aqueous solution and subsequent warming at a fixed temperature for facilitating crystallization prior to vacuum application for drying. The product has, however, been found to unavoidably contain traces of the amorphous CET-Na, which causes a rapid color development during storage. By using thermal analysis, differential scanning calorimetry (DSC), electric conductometry, and polarized light cryomicrographic techniques, the solubilities in water, freezing point, eutectic point, and melting behavior of CET-Na in aqueous solution were investigated. The investigation demonstrated that CET-Na in supersaturated aqueous solution is very stable, and that seeding with microcrystalline CET-Na to the supersaturated solution and subsequent cooling of the mixture till its freezing point gives neither any evidence for crystallization nor for growth of the seed crystals. The freeze-drying of CET-Na in the supersaturated solution after seeding has been demonstrated to give crystalline CET-Na containing neither of amorphous nor of quasi-crystalline form.