The polymorphism of azlocillin sodium (AZ) has not been discussed sufficiently in the literature. The results of infrared (IR) spectroscopy, thermal analysis (combined thermogravimetry TG and differential analysis DTA), and scanning electron microscopy (SEM) confirm that recrystallization of lyophilized azlocillin sodium from simple solvent acetonitrile causes polymorphic transformation. The new polymorph obtained by us has a crystalline form.