Using anti-chicken brain neurofilament antisera, Alzheimer's neurofibrillary tangles from two patients with senile dementia were stained by immunofluorescence and by the peroxidase-antiperoxidase procedure in cryostat sections of hippocampus and frontal cortex. In sections of cerebellum obtained from the same patients, the distribution of immunostaining was the same as that observed in experimental animals: Purkinje cell baskets and nerve fibers in the inner half of the molecular layer were demonstrated selectively. The immunostaining of the tangles was abolished when the antisera were absorbed by their own antigen, by bovine brain filament preparations, or by the fraction of bovine brain filament preparations nonabsorbed on anti-glial fibrillary acidic (GFA) protein immunoaffinity columns. Absorption with a bovine microtubule preparation isolated by two cycles of the assembly-disassembly procedure did not abolish the staining. Immunostaining experiments conducted on bovine brain filament preparations resolved on sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the antisera staining the tangles reacted with the 200,000-, 150,000-, and 70,000-dalton neurofilament polypeptides. Antisera raised to the 150,000- dalton bovine neurofilament polypeptide isolated by sodium dodecyl sulfate- polyacrylamide gel electrophoresis stained the tangle much less intensely, although Purkinje cell baskets in the cerebellum appeared well stained. No staining of neurofibrillary tangles was observed with antisera to other classes of 10-nm filament proteins (GFA protein, vimentin, and desmin).