We performed immunocytochemical staining to study the distribution of serum protease inhibitors in cerebral and systemic amyloid deposits. In beta-protein amyloid deposits in Alzheimer's disease, Down's syndrome, age-related cerebral amyloidosis, sporadic cerebral amyloid angiopathy and hereditary cerebral hemorrhage with amyloidosis of Dutch origin, antibody to alpha 1-antichymotrypsin (ACT) stains senile plaques and vascular deposits. Immature plaques or preamyloid deposits, identified by their positive staining for beta-protein and negative staining for Congo red, which represents the earliest recognizable stages of amyloid deposition, are also labeled. We did not detect ACT in other chemically different forms of cerebral and systemic amyloid. None of the other inhibitors in this study, i.e. antithrombin III and alpha 2-macroglobulin, was detected in the amyloid deposits. Neurons and glial cells throughout the central nervous system in normal and amyloid-containing brains also bind ACT antibody. The results emphasize the close association of ACT with one type of cerebral amyloid (beta-amyloid diseases) as well as the failure to detect such an association in other chemically different forms of cerebral and systemic amyloids.