Experimental allergic neuritis was induced in male inbred Lewis rats immunized with myelin, P2 alone, P2 mixed with galactocerebroside and P2 mixed with glucocerebroside. Neurological deficit started significantly earlier in myelin-immunized rats than in P2-immunized rats. Although myelin-immunized rats appeared most severely affected, differences between the groups in maximum neurological deficit were not significant. The course of the disease of the P2-galactocerebroside-immunized animals did not differ from that of the P2-glucocerebroside-immunized group. Histologically, cellular infiltration and demyelination were more conspicuous 12 days after immunization in the group immunized with myelin than in the other rats. After 21 days, primary demyelination was prominent in all groups: its frequency and severity were similar in the myelin-immunized and P2-immunized animals. The P2-galactocerebroside-immunized group had significantly more frequent demyelination than the P2-immunized group. Wallerian degeneration was prominent in all groups at this stage. We conclude that P2 alone does induce demyelination and that galactocerebroside added to the immunizing emulsion enhances the response but no more than the non-myelin lipid glucocerebroside.