Lewis rats were maintained on diets which provided either an adequate or deficient concentration of biotin. Biotin deficiency produced a marked reduction in thymus size and cellularity, a depressed immune response to sheep erythrocytes and prevented the development of experimental allergic encephalomyelitis following immunization with guinea pig myelin basic protein. Total T cells, T-helper and T-suppressor cells were quantitatively the same in the spleens of rats fed biotin-adequate or deficient diets. By using an adoptive lymphocyte transfer procedure, it was determined that the afferent immune response to myelin basic protein was impaired in biotin deficiency. These results indicate that experimentally induced autoimmune disease is susceptible to the nutritional influence of dietary biotin.