Oligodendroglial microtubular masses (OMMs) were identified immunohistochemically in brain tissue from patients with such degenerative disorders as Alzheimer disease, Pick's disease and argyrophilic grain dementia. The OMMs were identified by Alz-50 and antibodies to Tau-2, paired helical filaments and ubiquitin. They occurred preferentially in white matter. They were composed of fibers 25 nm in diameter and were localized to the cytoplasma of oligodendroglia and processes of oligodendroglia ensheathing damaged axons. They sometimes occurred in oligodendroglia also stained by antibody to the complement protein C4d. These data suggest that OMMs represent abnormal microtubular accumulations in oligodendroglia, possibly as a reaction to axonal damage.