a1-Antichymotrypsin (AACT, Alpha1-ACT) is an early-stage acute-phase plasma protein and a serpin that preferentially inactivates chymotrypsin, cathepsin G, and chymase. a1-Antichymotrypsin, a serine protease inhibitor, is tightly associated with amyloid plaques in Alzheimer’s disease (AD) and in normal-aged human and monkey brain. Regulation of the serine proteases and serine protease inhibitors plays an important role in neuromuscular differentiation. Prostate specific antigen (PSA), a chymotrypsin-like serine protease, is predominantly complexed to a1-Antichymotrypsin. A higher proportion of serum PSA is complexed to a1-Antichymotrypsin in prostate cancer than in benign prostate hyperplasia.