Acute experimental allergic encephalomyelitis (EAE) was induced in Hartley guinea pigs and Lewis rats, which were then treated with synthetic serum thymic factor (FTS). When a dose of 30 micrograms/100 g body weight of FTS was subcutaneously administered to the animals on days--1 (before inoculation), 4, 9 and 15 intermittently, clinical symptoms of acute EAE were suppressed. Histopathological evaluation showed that the severity of EAE in FTS-treated guinea pigs was less than in untreated guinea pigs. Immunohistochemical examination showed that the numbers of OX6+, W3/25+, W3/13+ and OX19+ cells in FTS-treated rats were less than in untreated rats and that the number of OX8+ cells in FTS-treated rats was greater than in untreated rats. These findings suggest that FTS induced OX8+ cells in inflammatory lesions and suppressed inflammation in acute EAE.