The epsilon4 allele of apolipoprotein E (apo E) is increased among patients with sporadic or familial Alzheimer's disease (AD). We examined platelet phospholipase C (PLC)-delta1 activity in AD patients either homozygous for apoepsilon3 or having at least one apo epsilon4 allele. We found that platelet PLC-delta1 activity is reduced from control levels in patients homozygous for apo epsilon3, but not changed in patients with an apo epilson4 allele. Reduced PLC-delta1 activity and apo epsilon3 may contribute to AD pathogenesis apart from the apo epsilon4 allele.