2-Imino-1-imidazolidineacetic acid (cyclocreatine) is an analog of creatine. It is reported to exhibit antitumour effect in some transplanted human and rodent tumours in vivo. It is reported as an efficient substrate for creatine kinase. It is an anticancer and neuroprotective agent. The crystal structure of cyclocreatine has been studied by X-ray diffraction methods. It is reported to crystallize as a zwitterion in the monoclinic system.