Paired helical filaments (PHFs) isolated from Alzheimer's disease (AD) brains were analyzed using freeze-drying/rotary shadowing and immunoelectron microscopy. These filaments are the major contributors to the formation of neurofibrillary tangles (NFTs) found in AD, and are composed primarily of the microtubule-associated protein tau. We have focused on the identification of PHF-tau protein within isolated PHFs using anti-tau antibodies (tau 14, 46, 60). These PHFs consisted of both helically twisted and "straight" paired filaments. With freeze-drying/rotary shadowing, we are able to demonstrate in 2-D and 3-D subtle twists within the straight filaments as well as immunolabeling of the individual filamentous strands composing the PHFs. Additionally, projections emanating from numerous filaments and bridges between PHFs often were immunolabeled with anti-tau antibodies. Our results suggest that tau proteins are present in discrete, nonconfluent patterns within the PHFs and are components of both strands composing the PHFs. Tau proteins are present in the bridges between individual PHFs and may contribute to interconnecting PHFs into a complex macromolecular network such as the NFTs found in AD.