Antisera to isolated Alzheimer neurofibrillary tangles (ANT) of paired helical filaments ( PHF ) were raised in rabbits. These anti- PHF sera immunolabeled both ANT in sections of Alzheimer hippocampus and ANT which were isolated and extracted with sodium dodecyl sulfate (SDS). The immunostaining of ANT in tissue sections was removed by absorption of the anti- PHF serum with small amounts of PHF and also with 40-fold the amount of a fraction prepared identically from normal brain; neurofilament and brain microtubule preparations used at the same concentration as the normal brain control fraction did not eliminate the tangle staining. Furthermore, the tangle staining was also not removed with glial filaments or actin and myosin filaments. No labeling of the neurofilaments of axons and cerebellar basket fibers by anti- PHF sera was observed in tissue sections from non-neurologic brain. On paper blots of SDS-polyacrylamide gels anti- PHF serum reacted with neither polypeptides of the normal brain control fraction nor major microtubule and neurofilament polypeptides. However, the immunoblots of PHF preparations with the anti- PHF serum revealed staining of several polypeptide bands in the 45,000-70,000 molecular weight (MW) region, material on top of the gel and diffuse staining of the high MW region. The tangles staining in tissue sections by the anti- PHF serum was abolished by its absorption with PHF polypeptides extracted from high and low molecular weight areas of SDS polyacrylamide gels but not with identically prepared neurofilament polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS)