By employing monoclonal phosphorylated neurofilaments antibody, we studied the abnormally pale-staining neurons in 3 cases of corticonigral degeneration by the avidin-biotin immunoperoxidase method. For comparison, the central chromatolysis in anterior horn cells secondary to cervical spine fracture and "ballooned" neurons in a case of Pick's disease and a case of Alzheimer's disease were studied with similar procedure. Achromasic neurons in the case of corticonigral degeneration and ballooned neurons in Pick's disease and Alzheimer's disease showed positive immunostaining, while neurons with central chromatolysis secondary to axonal injury did not. Our observations show that the achromasic neurons in corticonigral degeneration contain phosphorylated neurofilaments which share common antigenic characteristics with ballooned neurons in Pick's disease and Alzheimer's disease. The absence of positive immunostaining in reactive central chromatolysis suggests that despite the similarities in appearance with the usual histopathologic stains this cytoplasmic change is pathogenetically different from that in the other neuronal disorders mentioned above.