The structure and function of platelets in stored blood were tested for 21 days of storage with both light and electron microscopy as well as determination of platelet aggregation response to adenosine diphosphate. Observations were made for a period of 21 days of storage at 4 degrees C. These observations demonstrate the appearance of spontaneous platelet aggregates in stored blood which differ morphologically from aggregates induced by the addition of adenosine diphosphate during the first ten days of storage. The appearance of spontaneous platelet aggregates paralleled a decrease in platelet count and progressive decline in the number of morphologically normal appearing platelets. The platelets that were induced to aggregate with adenosine diphosphate formed more closely packed aggregates that those that formed spontaneously and that the adenosine diphosphate-induced aggregates could still be induced through the tenth day of storage, at which time few morphologically normal platelets could be clearly identified in the plasma. Spontaneous platelet aggregates showed no fibrin in the microaggregates but rather significant numbers of leukocytes contained in the aggregates which degenerated progressively beyond a storage period of five days.