Serum levels of alpha 1-antichymotrypsin (alpha 1-ACT) were measured in patients with early and late onset Alzheimer's disease (e-AD, 1-AD), patients with vascular dementia (VD) and healthy elderly. Patients with 1-AD were divided into two groups, one had normal alpha 1-ACT values and one had increased serum levels of alpha 1-ACT. Other acute phase proteins were also measured. The serum levels of alpha 2-macroglobulin (alpha 2-MG), alpha 1-antitrypsin (alpha 1-AT), ceruloplasmin (CER), transferrin (TRSF) and alpha 1-acid glycoprotein (alpha 1-ac.GL) were within the normal range. The C reactive protein (CRP) was occasionally detectable at low concentrations in e-AD, in both groups of 1-AD patients and in VD patients. Low serum concentrations of interleukin-6 (IL-6) were found in a higher proportion of 1-AD than in patients with e-AD or VD. These results indicated that increased levels of alpha 1-ACT along with occasional detection of IL-6 might be peripheral markers of the 'acute reaction' in the brain.