Effect of prednisolone on experimental allergic neuritis (EAN) in Lewis rats was studied. In order to exclude the influence of residual antigens, we evaluated the effect in EAN passively transferred with living lymphoid cells. Prednisolone, when administered after the passive transfer, prevented EAN both clinically and histologically, but when administered after the clinical onset, it had little influence on the course of the disease. Prednisolone was suspected to have suppressed the sensitized cells, but to have had no remarkable effect after the clinical onset.