Amyloid precursor protein (APP) is expressed by many non-neural tissues and it is possible that over-expression of the APP gene in non-neural tissue is responsible for the deposition of amyloid beta-protein in the brain and elsewhere. One possible source of beta-protein is circulating mononuclear blood cells which have previously been shown to express APP. To test this hypothesis, RNA was isolated from the mononuclear blood cells of patients suffering from Alzheimer's disease (n = 27), Down's syndrome (n = 13), senile dementia non-Alzheimer type (n = 14) and from normal individuals (n = 48). The relative abundance of mRNA coding for different splicing variants of the amyloid precursor protein (APP) mRNA was measured using multiprobe oligonucleotide solution hybridisation (MOSH). There was no significant difference in APP mRNA levels between any of the groups. This indicates that Alzheimer's disease is not characterised by an increase in production of APP in circulating mononuclear blood cells.