Kunitz-inhibitor containing forms of the beta-amyloid precursor protein (beta APP), known also as protease nexin II (PNII), and alpha 1-antichymotrypsin (alpha 1-ACT), a serpin, are important components of the serine protease and inhibitor balance in many tissues. In the nervous system, this balance may have trophic or growth factor activity at different stages of development, after injury and in disease states. In the current study, using immunocytochemistry and Western blotting with antibodies against the human homologues, we analyzed whether denervation affected the localization of beta APP and alpha 1-ACT in adult mouse muscle following axotomy. In mouse muscle, anti-human alpha 1-ACT antibody detected a 60 kD immunoreactive band and anti-human beta APP antibody a band at 92 kD in both normal and denervated extracts. beta APP was present in normal mouse muscle at both neuromuscular junctions and within intramuscular nerves. alpha 1-ACT was also detected at neuromuscular junctions, on the perineurium and endothelial cell surfaces. Following axotomy, both beta APP and alpha 1-ACT disappeared from intramuscular nerves simultaneously. However, at the neuromuscular junction, alpha 1-ACT decreased more rapidly with beta APP lingering before disappearing. Since both alpha 1-ACT as well as beta APP are present within senile plaques in Alzheimer's disease brains such experiments with the nicotinic, cholinergic neuromuscular synapse in denervated muscle may help to focus experiments on the mechanism of synapse loss as well as plaque deposition in this disease.