Studies of the metabolism and function of the amyloid precursor protein (APP) and its proteolytic fragment A beta in cultured cells, transgenic mice, and post-mortem brain tissue have advanced our understanding of Alzheimer disease (AD). However, the molecular pathogenesis of the disease is still not clear, and we are a long way from finding a cure for the disease. Studies carried out on human platelets and leukocytes have also helped shed light on APP and A beta metabolism and function. Platelet and leukocyte APP isoforms are processed using mechanisms similar to those in neuronal cells to generate A beta and soluble forms of APP. The activation of platelets and leukocytes leads to the secretion of APP and A beta, resulting in higher levels of these proteins in serum. APP and A beta in the circulation may be involved in the regulation of platelet function and in the modulation of immune responses. Because human platelets and lymphocytes produce all forms of APP and secrete amyloidogenic A beta peptides, these tissues may be useful in monitoring responses to therapeutic interventions directed at APP metabolism. Although not of neuronal origin, further studies on the more accessible ex vivo tissues, including platelets and leukocytes and other blood components, may reveal potential peripheral markers for AD and will further our understanding of the molecular pathogenesis of AD.