a,-Antichymotrypsin (al-ACT) is an early-stage acute-phase plasma protein and a serpin that preferentially inactivates chymotrypsin, cathepsin G, and chymase. Using immunofluorescence with four rabbit polyclonal and two monoclonal specific antibodies against human a,-ACT, we have localized a,-ACT at human and rat neuromuscular junctions (NMJs). Strong a,-ACT immunoreactivity (IR) was present at all NMJs identified by bound a-bungarotoxin (a-BT). a,-ACT immunoreactivity typically extended slightly deeper into the muscle fiber than a-BT, and it closely co-localized with immunoreactivities of post-synaptic desmin, beta-amyloid precursor protein, and dystrophin at the same doubleor triple-labeled NMJs. Topography of a,-ACT-IR was the same at human and rat NMJs. The muscle non-junctional sarcolemma was either not immunoreactive or was only very slightly so. When the primary antibody was omitted, absorbed, or replaced by a non-immune serum, there was no immunostaining. Thus, a,-ACT is a novel component of the NMJ. Although its role in the postsynaptic domain of the NMJ is unknown, it might be involved in the interaction between the presynaptic and postsynaptic components andlor inhibit excessive or unwanted serine proteases that may exist in the region of the NMJ. o 1994 Wiley-Liss, Inc.