The morphology of groups of platelets aggregated by means of an extract of the venom of the snake Trimeresurus okinavensis was contrasted with the appearances which occur when platelets aggregate to form an artificial thrombus in Chandler’s apparatus. The adhesion of platelets induced by venom extract illustrated many of the features usually seen at an early stage in the aggregation of platelets in plasma except for the absence of the formation of fibrin. The form and content of platelet profiles within the aggregates were readily distinguishable. Two platelet aggregates, one containing 43 profiles and the other 123 profiles, each typical of a type of aggregate, were taken for close study. The majority of platelet profiles in section which were greater in size than 1 µm<sup>2</sup> contained granules and in general those that were smaller than this did not. No granules were seen external to the aggregates. It is suggested that granule-free profiles of platelets represent peripheral processes from a granule-rich core cytoplasm and that as such these platelets are not ‘degranulated’.