Ten patients undergoing open-heart surgery received postoperative autotransfusion of shed mediastinal blood collected in the cardiotomy reservoir. The number, function and morphology of the platelets found in the shed blood were investigated. Platelets were counted using an electronic counter compared with light microscopy. Morphology of platelets was studied with electron microscopy. Platelet aggregation was studied using an aggregometer. Dense granule secretion was measured as the extracellular appearance of adenosine triphosphate. Enumeration of platelets in shed blood using the two methods gave different results. Thus, the electronic counter gave a mean platelet count of 62 x 10(9).l-1, while light microscopy revealed only a mean platelet count of 10 x 10(9).l-1. Electron microscopy disclosed few platelets, but numerous cytoplasmatic fragments smaller than or up to the same size as platelets. The platelets found were mostly shape-changed, spheroid, characterized by centralization and loss of alpha-granules and dense bodies, all changes that indicated irreversible platelet activation. The platelets failed to aggregate in response to the presence of thrombin, adenosine diphosphate or collagen, and secretion of adenosine triphosphate was absent. Plasma from the shed blood was not capable of inducing spontaneous aggregation in platelet-rich plasma from healthy donors. These results indicate that infusion of larger volumes of autotransfused blood should be supplemented with platelet concentrates.