Giant granules are uncommon in normal human platelets but are frequently observed in platelets from patients with the Chédiak-Higashi syndrome, some case of May-Hegglin anomaly, and individuals with chronic refractory anemia (preleukemia) or myelomonocytic leukema. However, the numbers of platelets containing the giant granule anomaly are so low that it has been impossible to isolate them in sufficient quantity to identify the mechanisms involved in their formation. Recently conditions were found that foster the long-term preservation of functional platelets in vitro. Ultrastructural examination of samples of C-PRP stored for 15--21 days revealed the presence of giant granules in 10--20% of the cells. Although factors involved in large organelle development were not determined in the present study, the new in vitro system for platelet storage may provide a useful approach for investigation of giant granule formation in normal and abnormal platelets.