An early-onset, familial form of Alzheimer's disease (AD) has been reported to be linked to a locus on the long arm of chromosome 21 (21q21). Furthermore, duplications in the vicinity of this locus involving the beta-amyloid gene and the proto-oncogene ets-2 have been reported in association with AD. The structural gene for Cu,Zn superoxide dismutase, SOD-1, is located between the beta-amyloid gene and ets-2. For this reason and because SOD-1 is a plausible candidate for a gene that might influence the fluidity of cellular membranes, we determined whether or not the subtype of AD with increased platelet membrane fluidity was associated with an increase in Cu,Zn superoxide dismutase activity.