Chronic relapsing experimental allergic encephalomyelitis (EAE) was induced in young Dunkin Hartley guinea pigs by a single sensitization with guinea pig spinal cord homogenate. The effect of either newborn thymectomy or young adult thymectomy on the clinical course of chronic and relapsing EAE was studied and evaluated statistically by Student's t test. All the animals that underwent thymectomy showed a significant delay in the onset of the disease compared with control groups. In addition, in the young adult guinea pigs in which thymectomy was done, the incidence of clinical disease was decreased. No substantial differences, however, were observed in the severity of the disease and number of remissions or relapses between guinea pigs in which thymectomy was done and in which clinical symptoms developed and controls.