Experimental allergic encephalomyelitis (EAE) is an organspecific, cell‐mediated inflammatory autoimmune disease and is regarded as a model of multiple sclerosis (MS). Relapsing EAE was induced in Lewis rats and the effect of mizoribine on the relapsing EAE was examined clinically, histologically and immunohistochemically. Mizoribine (4‐carbamoyl‐1‐β‐D‐ribofuranosyl‐imidazolium‐5‐olate), an immunosuppresive agent, is an imidazole nucleotide isolated from a culture infiltrate of Eupenicillium brefeldianum M‐2166. Most control and treated rats suffered two attacks. Treated rats at a dose of 10mg/kg per day showed clinically significant delay of the disease attacks and histologically reduction of infiltrating cell number. Treated rats at a dose of 20 mg/kg per day showed a further delay in the attacks and attenuation of clinical signs, and a smaller number of inflammatory cells and Ia positive cells were revealed at the first attack. This study suggests that mizoribine suppresses the clinical severity and inflammat