Abstract Platelets, when released as anuclear cells by their precursor megakaryocytes, already carry soluble proteolytic fragments of the amyloid precursor protein (APP) within their α-granules and intact APP in the α-granule membranes. In response to activation signals elicited by physiologic stimuli such as thrombin, platelets release their granules' soluble contents and translocate granule membrane-bound proteins to the plasma membrane. Because platelets carry >90% of the circulation's APP, activated platelets have been implicated as origins of the β-amyloid peptide fragment of APP (Aβ), whose deposition in the cerebrovasculature is characteristic of Alzheimer's disease. We have therefore studied the APP contents and proteolytic processing in resting DAMI human megakaryocytic cells, along with the consequences of the activation of these cells by thrombin, comparing the results in each case to those with human platelets. Resting and PMA-differentiated DAMI cell contents were examined by Western blotting, immunoprecipitation, or metabolic labeling with sulfur 35-labeled methionine during culture, while plasma membrane-bound APP was evaluated by flow cytometry. Activation was followed by changes in cytoplasmic calcium concentration ((Ca ++) in) and in membrane potential. Like platelets, DAMI cells exhibited a thrombin dose-dependent Δ(Ca ++) in, and membrane potential change; in contrast to the surface of a platelet, the surface of an agranular resting DAMI cell expresses granule-membrane proteins (APP and CD63) that appear on platelets only after activation. DAMI cell culture with 35-labeled methionine confirmed that megakaryocytes synthesize large amounts of APP, of slightly higher molecular weight, and degrade their APP extensively before platelets are formed.