Experimental allergic encephalomyelitis (EAE) was induced in guinea pigs by sensitization of myelinomas of homologous or heterologous (rabbit) brain in the complete Freund's adjuvant. Antilymphocytic serum (ALS) obtained by immunization of rabbits with the lymphocytes of the lymph nodes of guinea pigs was injected subcutaneously for 3--6 days at the early periods before or after the sensitization. ALS produced a marked inhibitory action on the development of the neurological signs of EAE and of histological changes in the form of demyelinization and perivascular cell infiltration in the lumbo-sacral and cervical portions of the spinal cord. Administration of ALS beginning from the 3rd--5th day before the sensitization or from the date of the EAE induction proved to be the most effective. When the injections were started from the 7th or the 12th day after the sensitization an increase in the incidence of the disease occurred in the grooups. A fall of morbidity correlated with a reduction of percentage of positive skin tests in response to the intradermal injection of a homologous myelin. No reduction in the production of the complement-fixing, antibodies to the heterologous antigen was seen in the animals given ALS. This pointed to the participation of cell factors in the development of the pathological process.