We have studied MHC class II antigen expression and lymphocytic infiltration during dopaminergic neurone degeneration produced by intoxication with 1-methyl-4-phenyl-1,2,3,6-tetrahydropiridine (MPTP). Microglial activation was observed in the striatum and in the substantia nigra (SN) in this model. We noticed a marked increase of MHC class II antigen expression on microglia and T-cell recruitment in these regions after MPTP treatment. B-lymphocytes were not observed. T-cell infiltration predominantly consisted of CD8+ cells at every time point but CD4+ cells were present too. More than a half of the observed lymphocytes showed strong staining of CD44 antigen. Our findings suggest a possible immune system involvement in the pathological process following MPTP intoxication.